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Gradience and categorical indeterminacy: determiner, pronoun, adjective, adverb or what?

What we are discussing here goes under different names.

Gradience or Categorical Indeterminacy

refers to the fact that it is sometimes difficult to say definitely to which class of words an item belongs.  It is usually a straightforward matter to assign words to word classes by looking at the functions they perform grammatically.  Nouns and noun phrases operate as subjects and objects, they are linked to other nouns and noun phrases by conjunctions, prepositions act to connect verb phrases to nouns and so on.
However, if we take, for example:
    I've just made tea.  Would you like some?
we have a problem with the word some.
Is it acting as a determiner to modify the absent (or ellipted) noun tea or is it acting as a pronoun to stand for the noun tea?
Gradience is often called categorical indeterminacy.  In other words, we cannot determine in principle to which category to assign to a word or phrase.
There are examples of this below.

Syntactical homonymy

is an allied but slightly different concept insofar as it refers not to indeterminacy of word class but to the fact that certain words and phrases may be performing slightly different functions depending on the speaker's intention.
Lexical homonymy describes the phenomenon of a word looking and sounding the same but carrying two unrelated and different meanings.  For example, in:
    They chased their quarry across the valley
and
    They got the stone from the quarry across the valley
the word quarry looks and sounds the same but its two occurrences are unconnected derivationally and semantically.  They are homonyms.
Syntactical homonymy refers to a parallel phenomenon in which an item carries a different meaning depending on how it is used in syntax.
For example, in this:
    He spoke, by the way, about his brother
it is in principle impossible to determine whether the phrase by the way modifies how he spoke (as an aside) or whether it is intended to modify the whole sentence and means that the speaker wishes the utterance to be taken as departing from the main topic of the conversation to which he or she will return.
In the first case, the phrase is an adjunct modifying the verb and in the second it is a subject-switching conjunct.
In both cases, the phrase is an adverbial so there is no difficulty in assigning it to that category.  The issue lies in deciding what sort of adverbial it is.  Only an understanding of the entire discourse in which it occurs can lead us to a conclusion about that.
If we front the phrase, as we usually do with conjuncts, the situation become slightly clearer.  In:
    By the way, he spoke about his brother
the phrase would usually be identified as a conjunct signalling a departure from the topic of a previous clause or sentence.
Adverbs, in particular, can function as adjuncts, modifying the verb phrase and integral to clause or as disjuncts or sentence adverbials indicating the style or attitude of the speaker.  For example, in:
    He smiled wickedly at her
we have an adjunct, wickedly, telling us how he smiled, but in:
    Wickedly, he smiled at her
we have the same word functioning as a an disjunct and expressing the speaker's emotional reaction to what was done.
For more on this, see the guide to adverbials, linked below, and the issue is also discussed in the guides to disjuncts and conjuncts, linked from there.

This guide concerns the problems one encounters when attempting to assign words and phrases to specific classes: categorical indeterminacy or gradience.  The problem is especially obvious with the catch-all adverb category.  Gradience in language is a polite way to say we don't really know what sort of a word this is, categorical indeterminacy is even more polite.
We have eventually to accept that boundaries between word classes are sometimes fuzzy.

An allied issue is that different people will use different ways of categorising words at all.  Some, Huddleston et al, for example, distinguish coordinators and subordinators as separate word classes usually subsumed by others in the class of conjunctions.  Others will assign pronouns to their own class and others may see pronouns as a sub-set of nouns.
Many grammars still recognise demonstratives as a class of their own while just as many will assign them to the general class of determiners along with possessives, elsewhere assigned to the class of possessive adjectives, and so on.
There is no definitive right answer to all this, of course, because much depends on the focus of an analysis.
In this guide and generally on this site, nine main word classes are recognised and it is to these that we shall refer in what follows:

Of these, the first four are considered open-class content word classes to which additions are readily made and which, standing alone, carry a signification which members of the speech community can readily explain.  These four categories also, it seems, are common to most if not all human languages.
The second four are considered functional, closed classes to which additions are very rarely made and which, standing alone, defy clear definition and whose significance is only clear when they function in the syntax of the language.
The final class falls into a grey area because, although the words carry some kind of meaning in context, standing alone they do not and they also do not perform a distinct grammatical function.  We can, however, and frequently do, create new interjections to express our emotions.

Conversion through grammaticalisation

One of the roots of gradience is the general tendency in many languages, not just English, to convert content words (those which carry independent meanings such as nouns, adjectives, most adverbs and verbs) into functional or structural words which operate grammatically rather than semantically (such as determiners, pronouns, prepositions and conjunctions).
There are more examples and a little more explanation of the phenomenon in the guide to the roots of English, linked below.

The process, sometimes called bleaching, is apparent in many items which start life as content words are converted into functional words with the meaning, so to speak, bleached out.  Vestiges of the original meaning often remain in place in other environments.

• The word like was originally either an adjective (derived from the Old English gelic, meaning similar in form) or a verb (differently derived from the Old English lician, to please).
  It now performs numerous other functions in the language:
  • as a pro-form for nouns with similar characteristics:
        We took all the old tools and the like out of the garage
  • as a conjunction:
        He played like his life depended on it
  • as a preposition:
        She ran like the wind
  • (informally) as an adverb:
        He was like delighted, I reckon
  • as a meaningless filler:
        I was like, at the bus-stop when I, like, saw her first and she was, like, walking towards me and ....
• The word instead was originally (up until the middle of the 17th century) written as two words and formed a prepositional phrase, in stead, meaning in position.  The word stead (Middle English stede) was a noun meaning place or stability and in other Germanic languages came to mean a town or dwelling place.  The words (as it now is, rather than a phrase) is used as:
  • a prepositional phrase with of in, e.g.:
        She came instead of her brother
  • an ungradable adverb only barely connected with the original meaning as in, e.g.:
        If you don't want to go to the cinema what do you want to do instead?
  • a conjunct adverbial in, e.g.:
        She didn't do it herself; instead she got the neighbours to

The two-faced (or more) nature of some words

Some words have the characteristics of more than one word class so, for example:

• that is a demonstrative determiner which has a plural form (those) making it a bit like a noun because in English determiners do not usually inflect for number (as they do in many languages)
• little is a determiner (and sometimes a pronoun) which has comparative and superlative forms (less and least) making it work like an adjective.  (The guide to synonymy and related concepts, linked in the list of related guides at the end, has more on the uses of little and small as adjectives, determiners, adverbs and nouns.)
• In
      He is running
  the word running is clearly a verb but in
      Running is good exercise
  the word is operating as a noun but still looks like a verb (and retains its verbal meaning).

In this guide, we are specifically concerned with words that appear, by some definitions and in some environments, to be adverbs and then pop up again elsewhere as pronouns, determiners or even adjectives.

To see what the problem is, try assigning a word class to the items in black in these examples and then click on the to reveal some comments.

I have seen enough	Because the verb is transitive, this use of enough is the object of the verb and is a pronoun.  It can be replaced by any other pronoun or a noun or noun phrase proper:     I have seen that film     I have seen them
They have discussed this enough	In this sentence enough is acting as an adverb because it is modifying the verb discussed.  We can replace the word with something more obviously adverb-like:     They have discussed this frequently or an adverbial prepositional phrase     They have discussed this for two hours.
Did he give you much trouble?	Here the word much is modifying the mass noun trouble, so it is acting as a determiner, specifically a quantifier.
Did you pay much?	Here there is no following noun so the word much is acting as the object of pay.  It's a pronoun and can be replaced by a noun phrase such as a lot of money. An alternative analysis is that the noun, money, is simply being ellipted because both speaker and listener share the information so the noun is unnecessary.  In that case, much is still the determiner of an absent noun.
She doesn't much like him	Here, the word much is modifying the verb like so it's acting as an adverb, specifically an amplifier.  It could be replaced by a more obvious adverb such as particularly.
Is she coming, too?	In this case, the word too is acting as an adverb, modifying the whole verb clause.  Specifically, it is an additive conjunct.
That really is too much work	Here the word too is acting as an intensifying adverb modifying the determiner much. An alternative analysis is to categorise it as a pre-determiner.
He is too hasty	Here, finally, we get to the simple adverb use of too as an intensifier for the adjective hasty.
It's on the far side of the house	The word far is modifying the noun side so it's an adjective.
Did you come far?	This use of far is not adjectival.  It's an adverb expressing distance.
This is far more important	Here, the word far is an intensifying adverb which amplifies the force of the adverb more.
He was dressed like Batman Books like Lady Chatterley's Lover were once banned	Here, the word like is a preposition, linking the verb with the noun complement.  It carries the meaning of in that manner / way or such as. It maintains, however, adjective characteristics even in the preposition role so we allow, e.g.: She is very like her sister
He spoke to me like I was a child	Here, the word like is a conjunction meaning as if.  The difference is that it is not usually followed by a subjunctive form: He spoke to me as if I were a child He spoke to me like I was (?were) a child
Did you ever hear the like?	Here, the word like is a pro-form standing for something said or read.
It smelt sort of cheesy like	Here, the word like is an adverb modifying the adjective cheesy.
Only she came to the party	Here, the word only is a determiner modifying the pronoun.
She only came to the party	Here, the word only is an adverb telling us that she did nothing else at the party.
She came only to the party	Here, the word only is also an adverb telling us that she came to no other event.
She came to the only party	Here, the word only is an adjective modifying the noun party.

Of course, as we noted above, a much wider range of words can slide between classes in most languages so we can have, for example:
    It's a clean car
    Please clean the car
    They are rich
    The rich live over that side of town
    She's running in tomorrow's marathon
    She enjoys running
and so on.
With lexical items such as these, it is not usually too hard to work out the meanings and assigning the items to word-class categories.
For more on conversion, see the guide to word formation, linked below.

Gerund or Verb?

An exception here is the traditional distinction between a gerund (a verb acting as a noun) and the -ing form of a verb denoting its aspect.  The distinction is probably not sustainable although it may act as a rough-and-ready rule of thumb in the classroom.

A third distinction, in some analyses concerns the distinction between a verbal noun and a gerund proper.  The difference is betrayed by the fact that a gerund retains some aspects of a verb and can be modified in the normal way of verbs with an adverb so we allow:
    Driving so quickly was dangerous
but a verbal noun may not be so modified and we do not allow:
    *Happily fishing is his main hobby
However, both verbal nouns and gerunds may be modified by adjectives so we allow both:
    Drunken driving is dangerous
and
    Fresh-water fishing is his hobby

It is clear that in, e.g.:
    She is cutting the article out of the newspaper
we have a verb, cutting, which is in the progressive aspect, preceded by the primary auxiliary verb, describing her current action.
It is also clear in, e.g.:
    She pasted the cuttings into the book
we have a noun derived from the verb cut which is acting as the object of the verb paste.  It is clearly a noun because we can make it plural in the usual way and it is pre-modified by the determiner the.  It can also be modified by an adjective such as new, relevant or various as nouns are.
So far, so clear.
However, there is a fuzzy middle ground where the situation is not clear at all and gradience is apparent.
For example, in:
    I objected to her cutting the article out of my newspaper paper
we have the item cutting acting as a noun insofar as it is the object of the prepositional verb object to and it is preceded by a possessive determiner, her, but it is also clearly verbal in meaning because it was the action of cutting that disturbed me, not the cutting that resulted.
and in:
    I noticed her cutting the article out of the newspaper
we have true categorical indeterminacy because this can mean either:
    I noticed her as she cut the article out
in which case the item is verbal, or
    I noticed her action of cutting the article out
in which case the item is a noun acting as the object of the verb notice.
(For more, see Quirk & Greenbaum (1973:391) where the authors list 15 possible uses of an -ing form of a verb varying from the purely nominal to the purely verbal with much fuzziness in between.)

What concerns us more here are what happens when functional words slip across boundaries.
We'll look at a number of these, some of which were exemplified above and consider what it is that the words are actually doing.
Until we know that, of course, it's hard to teach them.

	too and very
very tired

Both these words signify a positive degree of something.  They can function like this:

As adverbs or adverbials:

1. modifying an adjective:
   1. She's too tired to play
   2. I'm very tired
2. but not modifying verbal participles so:
   1. we allow:
          They are very frightened
          They are too frightened to go
      because these are adjectival participle forms
   2. but do not allow:
          *It is very altered
      or
          *Conditions have been too changed
      because these are verbal participles.
3. modifying other adverbs:
   1. She drove too quickly
   2. They went too far
   3. They played the music very loudly
   4. They came very close
4. modifying a determiner:
   1. Too many people came
   2. Very many houses have been built here
   3. Too little time was spent on it
   4. Very little time was spent on it
5. but not modifying a verb (as most other adverbs certainly can) so we cannot allow:
   1. *He is very driving
   2. *They are too smoking
6. The word too can also act as a conjunct adding to what is being said:
       This is cheap.  It will be easy to repair, too.
   but, unlike most conjuncts, too cannot appear anywhere but at the end.
   Very cannot perform this function at all.

As an adjective:
The word very can function adjectivally to mean an extreme end of something or an exact identity
    This is the very bed she slept in
    He lives at the very bottom of the valley
Too cannot perform this function.

With pre-modification:
The word very cannot itself be pre-modified so we don't allow:
    *Rather very good
    *Somewhat very quickly
    *Far very urgently
but the word too can be modified so we allow:
    Rather too expensive
    Somewhat too fast
    Far too cheap
etc.

With post-modification:
The word too cannot be post-modified so we don't allow:
    *It was too expensive indeed
etc.
but the word very can be post-modified with another adverb, indeed:
    It was very expensive indeed

It is worth knowing that some languages do not distinguish at all between the concepts of too and very, using one word for both.

	enough
not enough money

This word is often contrasted with too.  The adverb too signals an excess but enough signals a sufficiency.  However, as we saw above, syntactically, the words function very differently.

The word enough functions as:

An adverb which always follows what it modifies:

1. modifying an adjective used predicatively when it follows the adjective directly
   1. we allow
          The holiday was cheap enough
          They were happy enough
   2. but with attributive use, the determiner splits from the adjective and the result is slightly more formal
          A cheap enough holiday
          She's a hardworking enough student
          We need some strong enough tape to hold it in place
2. modifying an adverb
   1. He drove quickly enough
   2. I have walked far enough for one day
3. modifying an intransitive verb use
   1. It hasn't rained enough
   2. They have talked enough about this

A determiner which comes before what it modifies:

1. modifying mass nouns:
   1. We don't have enough milk
2. modifying count nouns:
   1. Do we have enough chairs?

A pronoun with transitive verb uses:
    I have written enough
    They have spent enough

	much / very much
not much money

These would, on the face of it mean almost the same thing but, in fact, much operates somewhat differently from very much.

As adverbs:

1. modifying participle adjectives but this is rare and some feel rather formal:
   1. participial adjectives only
          She is very much admired
          Some much annoyed passengers
   2. not for non-participial adjective
          *They are much angry
2. modifying verbs
   1. before gradable verbs only
          I very much hope she will come
          I much regret telling her
   2. not with non-gradable verbs
          *She much accomplished it
          *I much broke it
   3. in the negative only with care and mind
          She doesn't very much care what you do
          I don't much mind him staying here
      but not:
          *I much care what she does
          *They very much mind the noise
   4. with non-gradable verbs the words only quantify and come after the object:
          I didn't damage it very much
          I don't go to the seaside much
   5. Only the adverb very much can come after object noun phrases so we allow:
          I respect her and Mary very much
      and do not allow:
          *I respect her and Mary much
      or
          *I respect very much her and Mary
      but before nominalised clauses, so we allow:
          I respect what she has achieved very much
      and
          I respect very much what she has achieved
      but not
          *I respect what she has achieved much
      or
          *I respect much what she has achieved
3. much and very much do not modify other adverbs:
       *Is it much far?
       *I sold it very much cheaply

As determiners:

1. in non-assertive forms
   1. we allow
          I haven't got very much time
          Haven't they had very much food?
   2. but not in assertive forms
          *I have got much time
          *They have had much food
2. only with mass nouns
   1. we allow:
          They haven't had much fun
          They didn't have very much entertainment
   2. but not
          *We haven't got much sandwiches
          *They don't have very much computers
3. much and very much can also act as determiners to modify the non-assertive no-series of pronouns:
       What did you say?
       Nothing very much
   so not quite nothing
       Who did he speak to?
       Nobody much
   so not quite nobody
   Similarly, because they are also non-assertive forms, the any-series can be modified this way:
       I didn't see anybody much
       I can't see anything very much
   The items cannot, however, modify the some-series because these are assertive forms so we do not allow:
       *Something much
       *Was somebody much there?
   etc.

For more on the distinction between assertive and non-assertive forms, see the guide, linked below.

As pronouns:

Both much and very much can be pronouns, as in e.g.:
    He didn't say very much
    I didn't give him much
but there's a problem of gradience here, too, because it is sometimes difficult to determine whether we have ellipted or omitted the noun phrase.
If the noun phrase has been omitted or ellipted, the word is acting as a determiner.
If the word is standing in for a noun phrase, it is a pronoun.
Compare, for example:
    A: How much milk have we got?
    B: Not much (milk)
where the noun milk is uniquely recoverable – nothing else can fill this slot in the sentence – so we have a true case of ellipsis of the noun and much / very much is still a determiner.
with
    A: What did he buy?
    B: Not (very) much
where it is clear that we are ellipting the subject and the verb phrase (He bought ...) but almost any verb phrase, noun phrase or nominalised clause is imaginable as the object of the verb:
    He bought a few vegetables
    He bought nothing
    He bought me a small gift
    He bought what he had always wanted to have hanging on his wall
etc.
so here we do not have a case of pure ellipsis (because the omitted item is not unique) and the use of much / very much is arguably pronomial.

	more and less
more accurate clocks

In the example above we could either assume that the reference is to:
    additional accurate clocks
or
    clocks which are more accurate
and there is no way without clear context that we could decide what is meant.
Additionally, we can interpret:
    She has less expensive clothing than her sister
as either:
    Her clothing is less expensive than her sister's
or
    She has fewer expensive clothes than her sister
and, again, with no context, we cannot decide what is meant and the reason lies in gradience.
Specifically, in this case, more and less can operate as adverbs modifying adjectives or as determiners relating to noun phrases.
Like this:
Take these example sentences:
    They avoided buying more expensive cheese
    They bought less expensive cheese
We can analyse the sentences in two ways:

As you see, the key lies with what the words more and less are doing.
In Analysis 1. the words are determiners.
The first sentence implies that
    They had already bought some dear cheese and avoided buying more of it
and the second sentence implies that:
    They had already bought some dear cheese but bought less of it thereafter
In Analysis 2. the words are adverbs modifying the adjective expensive.
The first sentence implies:
    They bought no cheese which was dearer
The second sentence implies
    They only bought cheaper cheese.
It is impossible to tell by looking at the sentences what the words more and less are doing grammatically.

These two words can also function as pronouns but in that case no ambiguity arises because the adverb form cannot stand alone.
    They avoided buying more
    They bought less
can only refer to quantity and the words more and less are pronouns allied to the determiners, not the adverbs.

The issue of ambiguity in deciding what is the noun phrase and what is the determiner is investigated more fully in the guide to constituent analysis, linked below.

	(by) far
how far!?

The word far is unusual in being both an adjective and an adverb.

As an adjective it is quite simple and the antonym of near:
    Put on the far wall
    It's on the far side of the moon
    The furthest corner of the room
It is quite rare to use the comparative when the word is adjectival:
    ?The further corner of the room
In adjectival uses, by far is not allowed.

As an adverb, the word has two distinct uses:

1. An adverb of distance but only in non-assertive contexts:
   We allow:
       It isn't far
       Is it very much further?
   but we do not usually allow:
       *It is far
   In this sense, far cannot be pre-modified with by.
2. An intensifying, amplifying adverb which can be emphasised by the addition of by, especially with superlative forms.  In this form it can:
   1. intensify a verb or phrase signifying preference:
          I would far prefer the red one
          I would far sooner have a rest
          They would far rather stop now
      In this sense, far cannot be pre-modified with by.
   2. intensify a comparative or superlative form of an adjective:
          That is far nicer
          That is far more interesting
          That is (by) far the best way to do this
          They are (by) far the most serious students in the group
      where the word can be emphasised with the addition of by.
      When far modifies the comparative it cannot usually be pre-modified with by:
          *That is by far nicer
   3. intensify comparative, but not usually superlative forms unless modified with by, of an adverb:
          That was far more persuasively expressed
          She drove far more quickly than I could have driven
          That was (by) far the most impressively presented paper
      the usual preference is for much in this form:
          That was much the most impressively presented paper
   4. intensify comparative and superlative forms of quantifiers:
          They have far less money than we have
          There are far fewer people here than I expected
          They have (by) far the most time of all of us
      but not:
          *They have far the least money
      for which, again, much is preferred and is rare
          They have much the least money
      or the by far term is compulsory:
          They have by far the least money

Some other examples of words that live on the borderlines

It is not just determiners, adverbs and pronouns that cause problems by existing on the borderlines between word classes.  Even within established word classes, some items switch between groups.  Here are some common examples of syntactical homonymy.

conjunct or conjunction?

Take these two examples:
    He is tired and getting old.  Yet he works a six-day week.
    I was tired yet happy with my efforts
In the first case, yet is an adverbial acting as a conjunct referring anaphorically to the first sentence.  It could be replaced with a more familiar conjunct performing the same function, such as however.
In the second case, it is a conjunction meaning something like but.
The word so also inhabits this murky area: sometimes a coordinating conjunction, sometimes a conjunct.  For example, in:
    I'm not going to work so I don't need the car
the word is acting as a coordinator of two main clauses (and the clause ordering cannot be reversed sensibly).
However, in:
    I'm not going to work and don't need the car.  So, you can use it if you like.
the word is acting as a conjunct linking the second idea anaphorically to the first.
Moreover, in:
    I came early so I can help
the word links the reason with the action and is a conjunction
but in
    I came early.  So, I can help
the word links the consequence, not the reason, and is a conjunct.
In spoken language, the second of these would be distinguished from the first by pausing after So, by intonation and phrasing.

adverbial, noun phrase or post-modifier?

Three more examples:
    He arrived with his sister
    That old man with the black dog
    The only way to shift this seems to be with a hammer
In the first we have an adverbial prepositional phrase, with his sister, telling us some about his manner of arrival (accompanied by his sister).
In the second case we have a prepositional phrase, with the black dog, post-modifying a noun phrase (that old man).
In the final case we have a copular verb connecting a noun with a non-finite verb phrase which in this case can be classified as nominal (because that's the normal non-adjectival subject complement in such sentences).  The phrase with a hammer is an instrumental prepositional phrase.

adjunct or disjunct?

Take these two:
    Politically, that's a suicidal idea
    Politically, the question is one of legitimacy
Usually, an adverb like politically, is classified as a viewpoint or angle adjunct (i.e., one which expresses the field of interest in which the comment is set).  In the second example, that is what it is doing.
In the first of these examples, however, it is clear that the speaker is signalling how the statement is to be understood and that is the job of attitude disjuncts, not adjuncts.

adjective or noun?

An example:
    The upright chairs will go well with the new table
    OK, I'll get the upright
The upright what?  If we call this ellipsis of the noun then the noun itself must be 'uniquely recoverable', i.e., there must be only one possible completion of the sentence.  Here, however, we could complete the sentence with chairs, ones, furniture, sort, sorts, type, types and a number of other nouns and pronouns.  It is not, therefore, simply a case of ellipsis.
It might be a case of just omitting a noun phrase, of course, but that phrase has to be something describable as upright and it's hard to see how we can have upright sorts as a meaningful phrase.
Perhaps it's best just to call the word a nominal adjective akin to something like the French, the old, the unhealthy etc.  In that case, it has ceased being an adjective at all and is now a noun.  That's how it appears here because it is modified with a determiner, the, and nouns get that treatment.  We could even make it plural.

adjective or preposition?

There is no problem assigning words such as at, in and on to the closed class of prepositions.  However, some prepositions are not so centrally prepositional and behave in two respects like or as adjectives.
They can have comparative and superlative forms so we allow:
    He was near the house
    He was nearer the house
    He was nearest the house
and the complex preposition close to works similarly.  The marginal preposition like works in the same way but is always made comparative periphrastically (with more and most):
    She is like her mother
    She is more like her mother
    She is most like her mother
Secondly, near and close can be used predicatively and attributively as real adjectives as in:
    His house is close
    Her house is nearer
    The nearest farm is over there
etc.

noun, adjective or classifier?

One example will do:
    He's an English teacher
This is, of course ambiguous.  If we stress the first word, English, we are referring to the subject taught and if we stress teacher, we are referring to his nationality.
In the second case, English is clearly an adjective although it doesn't behave like adjectives should in all cases, having no comparative and superlative forms for one thing.  We can call it a denominal adjective (i.e., one formed from a noun) and be done with it.
If we leave the stress on the first element, we still have a problem.  English is the subject he teaches and as such, it's a noun just like literature, physics or quantum mechanics.  It cannot, therefore be an adjective because we can treat it just like a noun by pre-modifying it with an adjective (advanced English, for example) or a determiner (some English) or a post-modifier (English spoken in Canada) and so on.
If we do that here we get the truly ambiguous an advanced English teacher or some English teacher etc.
So it's not a noun either, really.
We can fudge it and call it a classifier (which is just a special form of adjective also known as a noun adjunct) or we can ditch the adjective idea altogether and call it part of a compound noun.  If we do that, we can treat it as a single entity and have, e.g., an old English teacher, a small, hairy English teacher and so on.
What we can't do is shift the adjective and have a conversational English teacher so we are back to having teacher as a noun pre-modified by English, the adjective.
Take your pick.

adverb or preposition?

Take these:
    They went inside the house for dinner
    John came outside the house for a cigarette
    Mary came out of the dining room and joined him and they chatted outside for a while
In the first example, we have inside functioning as a preposition because it has a noun complement, the house.  It can't be an adverb because adverbs don't have complements.  We are OK so far.
The second sentence is also fine.  We have outside the house with the preposition outside having the noun complement the house.  We are still OK.
In the third sentence we have out of the dining room and we can analyse this two ways:
As a prepositional phrase expressing direction as in out of the blue, out of the car etc.
As an adverb, out, (as in she let the cat out) followed by a prepositional phrase saying where from rather than where to, of the house.
We could say Mary came out and leave it there, in which out is acting as an adverb, and we still have the modifying prepositional phrase of the house to tell us more about the verb come out.
It is trickier now because we do not know whether in the second use of outside we have simply ellipted the complement of outside (the house) so it stays a preposition or are using the word as an adverb (opposed to inside).
The final problem here is that out of functions as a preposition of place and a preposition of direction depending on where it appears
Place:
    He is never out of the house
    That's out of the question
    He left it out of the suitcase
etc.
Direction:
    They got out of the car
    It emerged out of the swamp
    She went out of her mind
etc.
Multi-word verbs are a trap for the unwary because so many of the particles can function as prepositions and as adverbs depending on how they are combining with other items in the clause.  These are better seen as examples of syntactical homonymy rather than gradience because it is in principle simple to determine the item's word class.
For example, in:
    He walked up the road
the word up is a preposition but in
    He woke up at six
the word up is an adverb and so it is in
    He turned up late
For more, see the guide to multi-word verbs, linked in the list below.

adverb, preposition or conjunction and what sort?

Another incidence of the adverb-preposition confusion is exemplified by the words for, since and during. and one reason the words cause trouble for learners is the distinction in their grammatical functions.
With time expressions, we are accustomed to teaching the words as prepositions with noun-phrase complements or objects:

1. during relates to an event or time span which occupies a stretch of time (which may, in fact, be quite short but not instantaneous).  Because the word refers to a completed time span, it is rarely used with a relational tense, such as the present perfect.
   It has two closely related functions:
   1. referring to the whole of a period (in its meaning of throughout), so we get, e.g.:
          He lived here during the sixties
          She taught here during the school year 2001-2002
   2. referring to some point in time in the period in question, e.g.:
          I left during the advertisement
          He felt unwell during the party
2. for is also used for a time span but not for an event so it is conventionally followed by a straightforward time expression.  The length of time is immaterial.  We get, therefore, e.g.:
       He stayed for 5 minutes
       He thought for a nanosecond
       She lived here for 60 years.
       She has worked there for a long time
   Because the reference can be to completed or uncompleted events and actions, both relative / relational tenses and absolute tense forms are applicable.
   (In terms of syntactical homonymy, the word is also a coordinating conjunction, with the meaning of the subordinating conjunction because, albeit used ever more rarely in modern English:
       I refrained from answering for I knew she would be angry)
   As a preposition for also has a number of other functions (at least 14) but these are not to do with syntactical homonymy because the word remains firmly a preposition:
   1.  list of 14 is available here (new tab):
          There's a letter for you
   2. Having the purpose:
          The house is not for sale
   3. Because of:
          He feels better for his long weekend break
   4. Amount of time or distance:
          He spoke for hours
          We drove for miles
   5. On the occasion of:
          I bought it him for his birthday
   6. Compared with something:
          He's not a bad cook for a man
   7. In support of:
          I won't vote for the President again
   8. In relation to someone or something:
          She's very nice but too old for me
          I have a liking for hot curries
   9. In exchange
          I traded in my old car for something a bit more reliable
   10. Representing
           I played football for my school
   11. In the direction of:
           We headed for Madrid
   12. Meaning
           What's the German for dormouse?
   13. To get or achieve:
           They waited for a bus in the rain
   14. Duty / responsibility
           It's for the manager to decide what happens next
3. since is also a problematic word because it functions as a temporal preposition, too.  It refers, however, to the following time span up until another event intervenes or until the present.  For example:
       She has worked here since October
       They had been in their jobs since the beginning of the year but left in October
       They have waited since the first meeting for an answer
   Unfortunately, the word is also a good example of syntactical homonymy because it can also function as:
   1. an adverb:
          I saw her when she came to visit her mother, but not since
          He started on a low salary but has since been promoted and now earns well
      and here, as the first example shows, the tense forms are variable (not, as is sometimes averred, always relative or perfect forms).
   2. a temporal subordinating conjunction:
          He had lived there since he came to London but moved when he retired
          I have been at university since I was 18
   3. a causal or resultative subordinating conjunction
          I had a drink in the bar, since I had an hour to kill before my train
          Since it was Sunday, I stayed in bed till noon

reduced relative clause, progressive tense or prepositional phrase?

Take this example:
    The price including materials is £400
In this sentence we can analyse the words including materials three ways.
It is a reduced relative clause and in full would be:
    The price that includes materials is £400
It is an example of ellipting is in a progressive tense form and in full would be:
    The price is including materials and is £400
It is a prepositional phrase and is the equivalent of:
    The price with the materials is £400

adjective or adverb?

1. Consider:
       He rolled it flat
       She cut them small
   In both these examples, we have an adjective (flat and small, respectively) but no copular verb to link the adjective to the pronoun.
   At first glance, they appear to be malformed sentences because verbs need adverbials to complement them so it should be:
       He rolled it until it was flat
   or
       She cut them into small pieces
   In fact, this is a perfectly acceptable use of adjectives playing at being adverbs.
   What we have is called a proleptic use and that means we are looking ahead to the result of the action rather than how the action was performed.
   That's the technical explanation but we still have adjectives acting as adverbs and sliding across word classes.
2. Adverbs of definite frequency, such as daily, monthly, hourly etc. function is the normal way you'd expect adverbs to behave.  However, they also function quite happily as adjectives so we can accept all of these and many more:
       We had a weekly meeting
       We met weekly
       They take stock fortnightly
       There's a fortnightly stock take
   and so on.
   We cannot, however, do this with annually or seasonally as those words only function as adverbs, formed from the adjectives annual and seasonal.
3. Less explicable are the words kindly and unkindly which are formed as adverbs from the adjective kind so we have, e.g.:
       She spoke kindly to the child and unkindly to her father
   However, both words also function as adjectives in, e.g.:
       Normally a kindly person, she was unkindly when angered
4. The word wide is also problematic because it acts as a normal adjective in, e.g.:
       a wide road
       the river was wide
   etc. but is a predicative-only adjective in
       His quotation was wide of the mark
   and as an adverb modifying another in:
       Plant them wide apart
   and as an adverb modifying a prepositional phrase in:
       Keep these children wide apart from the others
5. Three odd words – aplenty, akimbo and galore – always follow the noun attributively.  None can be used predicatively:
       We have food aplenty and drink galore
       *The food is aplenty and the drink is galore
       He stood with arms akimbo
       *His akimbo arms
   There is some lexicographical disagreement with these words.  Oxford dictionaries once classified all of them as adverbs rather than adjectives but have since decided that at least aplenty and galore are adjectives.  Cambridge dictionaries describe them as adjectives as does Merriam-Webster (although there's a bit of hedging there with akimbo described as either an adjective or an adverb).  The word aplenty has also (but not on this site) been described as a postpositioned quantifying determiner which makes two if we include enough in that category.
   It is difficult to see how any of these can usefully be described as adverbs because no substitute adverb is usually available so we do not allow:
       *He greeted me with arms openly
       *We had food plentifully
       *They had drink abundantly
   We can, however, replace them with adjectives and allow:
       He greeted me with open arms
       We had copious food
       They had abundant drink
   so, on this site, we stick with them being adjectives but that's not a certain categorisation because they are clearly sometimes adverbial in nature so they can be replaced with adverbial phrases as in:
       He stood with arms at his side
       We had food in large quantities
       They had drink in great amounts
   so, in these senses, the words are adverbial even if they aren't really adverbs.

The moral of the story

Do not classify words by what they look like or by what a dictionary tells you they are.  Look at what they are doing and teach them accordingly.
Languages use word class similarly because they all arguably exhibit elements of universal grammar such as nouns, adjectives, determiners, adverbs and so on.  The issue is how they do it and then we find a bewildering range of possibilities.  It won't help our learners if we are sloppy or careless about assigning word class indiscriminately, regardless of what a word is actually doing.

If, for example, our concern is to teach the use of a word like enough as a quantifying determiner (coming before what it modifies), then it will confuse learners if the word appears in a text in that role and either of its other roles as a pronoun or a post-modifying adverb.  The following is, therefore, difficult to use as a model because there are three uses of the word in a single short text:

A: Do we have enough money?
B: Not this month.  We have already spent enough on the new furniture.  I ought to ask for a pay rise, I guess.  I work hard enough, don't I?

Reference:
Quirk, R & Greenbaum, S, 1973, A University Grammar of English. Harlow: Longman

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