Types of modality

There is a simpler and much abbreviated essential guide to modality in the pre-service, initial plus section of this site and one to central modal auxiliary verbs. Both those links will open in new tabs.

What follows is an analysis which can be used when planning to teach modal auxiliary verbs, modal adjectives, adverbs and nouns in English. It is based on some fundamental concepts in logic and the theory is that the categories of modality mirror the categories of human thought, making concepts clearer and more teachable.

Propositions and Modality distinguished

At the outset, we need to distinguish between propositions and modality because that will help us to define what we are talking about.

If, for example, I say:
The door is red
I am not implying any sense of whether it should be red, must be red, can't be black or whatever. I am simply stating a proposition and the hearers will make of it what they will.
If, however, I say:
You need to use the red door
I am stating something very different. In effect, I am saying that there is some kind of obligation or duty on the hearer to use one door rather than another. In other words, I am stating my perception of the situation rather than stating a fact.
This guide is concerned with how English speakers express their feelings about events and states.

This is an important area because it is impossible to speak any language properly without knowing how to encode your feelings about events and states. Confining yourself to statements about the world only is akin to trying to write a novel without using the letter 'e' and is communicatively crippling.
Unfortunately, it's also a complex area and the complications are often underestimated.

There is, to muddy the water slightly, a distinction sometimes made between event modality and propositional modality. Luckily, the two types of modality can be simply explained:

Event modality refers to the perceived benefit of an event occurring. For example:
This work must be completed by August
It is important that the staff are kept informed
which both refer to how the speaker / writer perceives a benefit from an event.
Propositional modality refers to the speaker / writer's view of likelihood of a proposition being true. For example:
That's obviously his sister
She should be in Paris by now
which both refer to how likely the speaker perceives a proposition is true.

In the analysis used on this site, event modality concerns deontic modality and propositional modality concerns epistemic modality. For an explanation of those two terms, read on.

Modality lies in the realms of interpersonal meanings and expresses how the speaker / writer feels about a state or event: whether it is true, necessary, inevitable, possible, desirable and so on. Modality is a semantic phenomenon rather than a syntactical one because it is concerned with meaning and meaning can be realised through a wide range of structures.
All of the following are, therefore, expressions of modality of one kind or another:

I need something to eat
That is a necessary conclusion
We are unable to help
Water freezes at 0° Celsius
I have no obligation to pay the money
I am willing to help
They can't be late
I shouldn't have said that
Pass the salt

My conclusion is that she doesn't like me
You can't be serious
We must go
That is probably true
She is definitely related to Mary
Will you marry me?
I expect to arrive about 6
I managed to get it done
Catching that flight means getting up very early

You will see that only six of those examples contain a modal auxiliary verb (need, must, can (twice), should, will). In what follows, many examples of modal auxiliary verbs are used to exemplify the concepts but you should not assume that such verbs are the only way of expressing modality. The other examples above contain modal adjectives (necessary, willing, unable), modal nouns (conclusion, obligation), modal adverbs (probably, definitely), and lexical or main verbs used to express modality (freeze, expect, manage, pass, mean).

Views of modality

A traditional view

Traditionally, modality is taught in two main ways:

By taking modal auxiliary verbs or other expressions of modality and seeing what kinds of meaning they carry. This is a common analysis (used elsewhere on this site in the consideration of modal auxiliary verbs) taking each verb in turn and itemising and explaining what functions they can perform. For example, with the modal auxiliary verb might and other ways to express the meanings it can realise we get:

Function	Example
present possibility	Careful. There might be a snake in the hall. I think it's possible there is a snake in the hall
future possibility	It might rain tomorrow. The weather forecast is for rain tomorrow.
past possibility	He might have telephoned while I was out. It's possible he telephoned while I was out.
suggestion	You might try taking an aspirin. Perhaps an aspirin helps.
permission	Might I talk to you? Do you have a little time to talk to me?
complaint	You might have warned me! It is regrettable that you didn't warn me.

By looking at some ways of describing modality and see how we can realise the meanings in English. This means looking at concepts such as:
Ability:
    I can go
    I am able to go
Permission:
    You may go
    I allow you to go
Advice:
    You should go
    Going is advisable
Obligation:
    They must go
    They are required to go
Possibility:
    They might go
    Perhaps going is what they will do

However, both these approaches have drawbacks.

Approach 1.: can easily overwhelm learners because a single verb (might in this case) can express six different and unconnected meanings and other modal auxiliary verbs suffer from the same polysemous state so verbs like can and should are used in English to represent many meanings (ability, permission, deduction etc.) and there is no one-to-one relationship between the verb and its meaning.
Approach 2.: is unfortunately too loose because the categories do not lead to any sensible decisions about what they mean and they overlap.
For example, does:
    Off you go!
express permission or obligation?
Does:
    I can do that for you
express ability or possibility?
Does:
    He should be there
represent a deduction about a possibility or some advice?
Does:
    You must try some of this chocolate cake
mean I am giving advice or obliging you to do something?
and so on.

A different view

The traditional views have the advantage of being familiar and using terms readily understood by everyone but, for teaching purposes, they are flawed ways to do the analysis. Speakers of the language do not proceed from a consideration of the various functions of the verb, adjective, noun or adverb and then make meaning using it. What they do is to conceive a meaning and then select a modal expression which suits their purposes.

We need a better representation of modality which does two things:

Avoids the overlapping, loosely defined categories
Represents types of modality which bear a greater resemblance to how people think

With this in mind, we can try an alternative way to represent the diversity of modal auxiliary verbs and other modal expressions using adverbs, nouns, lexical verbs and adjectives in English.

For example, I may have in mind the fact that I do not believe Mary when she says:
    Hey, I saw a unicorn on my way back from the pub last night!
For my purposes, I might select any number of expressions of modality depending on my degree of certainty, formality, the setting, how well I know Mary, whether I owe her obedience and so on.
I may consider, for example:
    It's an impossibility that you saw a unicorn
    You can't have seen a unicorn
    If you say so, it must be true
    It is impossible that you saw a unicorn
    I suppose you might have seen a unicorn, but I doubt it
    You couldn't have seen a unicorn; they don't come round here
and so on.

These examples are forms of epistemic modality (i.e., to do with the speaker's view of the truth of a proposition) although the verbs and expressions themselves can be used to express other types of modality. They all express a degree of certainty about the central proposition.

4 shades of modality

epistemic modality: Epistemology is the study of theories of knowledge and the word epistemic means relating to knowledge.
This kind of modality is to do with the speaker's perception of the truth or otherwise of a proposition. In the loose traditional terms, it's to do with (im)possibility, (un)likelihood, (un)certainty etc.
So, for example, on hearing a knock at the door, the use of will in:
    That will be the postman
signals the speaker's certainty about the proposition.
Equally:
    That can't be the postman
signals the speaker's certainty of untruth and the use of might in:
    That might be the postman
signals the speaker's uncertainty.
Modal auxiliary verbs of deduction fall into this category (should, must, can, ought and more), as do expressions such as:
    I think that's the postman
    I don't believe that's the postman
    Do you reckon that's the postman?
    My feeling is that that's the postman
    It's impossible that it's the postman
    I suppose it's the postman
and so on.
deontic modality: Deontology is the study of duty and deontic means relating to obligation and duty (or its lack). In the loose traditional terms, it's to do with obligation, lack of obligation, advice, duty, prohibition, necessity etc.
For example, the use of ought to in:
    You ought to write to your mother
expresses the speaker's perception that it is the hearer's duty to write and the use of must in:
    You must not write like that
expresses the speaker's view that the hearer is obliged not to write in that way.
The use of may in:
    You may go
expresses the speaker's view that there is no longer a duty or obligation to stay.
Modal auxiliary verbs expressing any sense of obligation or its lack (should, must, can, may, might, ought and more), prohibition and advice fall into this category as do expressions such as:
    Your mother will be expecting a letter from you
    It's your duty to write
    Writing like that is unwise
    I'm telling you not to write like that
    Go if you like
    It's not necessary to stay
and so on.
dynamic modality: The term dynamic needs no explanation but here it refers to the fact that the modality is centred on the subject and encompasses ability and willingness. In the loose traditional term, it's to do with ability but the willingness aspect is often ignored or put into considerations of talking about the future (where it does not belong and has served to confuse and bewilder generations of learners and, alas, some teachers).
For example, the use of could in:
    I could swim well as a child
refers to the subject's own ability and not to externally imposed obligations or duties or any sense of likelihood or impossibility and the use of can in:
    Can you understand this?
refers to the hearer's ability alone and:
    I'll get it for you
refers to the speaker's willingness to do something (volition). It is not, incidentally, a future tense form and does not necessarily refer to the future at all; it is an expression of current willingness to do something. The modal auxiliary verbs expressing ability or willingness (will, would and can mostly) fall into this category as do expressions such as:
    I had the ability to swim well as a child
    I was able to swim well as a child
    Are you going to help me with this?
    I need some help with this
    I'm willing to get it for you
    Let me get it
and so on.
Dynamic modality is sometimes called personal modality because it applies, in most cases, to people's abilities and willingness.
alethic modality: This term derives from the Greek word for truth [αλήθεια, aleethia] and refers to logical necessity (rather than deduction which concerns epistemic modality). For example, the use of must in:
    A square must have four sides
refers not to the speaker's perception and not to any form of obligation, ability or deduction but to the fact that one of the necessary conditions of being a square is to have four sides and the use of can't in:
    Parallel lines can't meet
equally represents the truth of a proposition, failing which the lines cannot be parallel. A few modal auxiliary verbs (must, can't, will, mustn't and need mostly) fall into this category as does the present simple tense form of the verb and other expressions such as:
    A defining characteristic of squares is that they have four equal sides
    Lines which are parallel never meet
In a sense, this form of modality is the odd one out because it does not rely on the speaker's view of an event or state. Alethic modality is concerned with what is universally true in all cases independent of the setting.

Subtler shades

This area of study is infested with a menagerie of terminology, some of which is concerned with quite subtle levels of analysis.
So, if you come across alternative expressions, here's a very short guide:

Epistemic modality is often singled out as being views of the truth based solely on the evidence available to the speaker and separated in that way from alethic modality which is not contingent on the speaker's setting but expresses truths in all possible worlds. We make a distinction between the two types here, too, but should bear in mind that they overlap slightly so, for example:
This must be the answer
That can't be right
can express the speaker's certainty that it is correct or not or that the truth is an inevitable outcome of the laws of mathematics depending on the situation referred to.
Epistemic modality is sometimes further broken down, e.g., by Palmer, 2001, into:
- judgemental or speculative modality: referring to the speaker's judgement or understanding. For example:
  I think that's too much
  That might not be enough
  etc.
- deductive or evidential modality: referring to the speaker's view as influenced by evidence. For example:
  It might rain later
  I think it's going to be a difficult job
- assumptive modality: expressing the speaker's view of the truth based on previous experience. For example:
  She'll be at work at this time of the morning
  Don't call now. They'll be having dinner.
Deontic modality is sometimes distinguished from or broken down further as:
- boulemic modality: that which is necessary given a person's wants and desires. For example:
  I must go now and get something to eat
  You must not stay if you have work to do
  etc.
- jussive modality: an obligation placed on a third person. The jussive mood is also used in some languages with a distinctive verb form to express wishes about the actions of a third person. For example:
  He really should try to be more patient with the children
  Mary has to be the one to do it
  etc.
- commissive modality: an obligation placed on the speaker by the speaker. For example:
      I will pay tomorrow
      I'll get it done; promise
      I have to thank you
  etc.
- imperative or directive modality: an obligation placed on the hearer. For example:
  You should get some rest
  This has to be done before we can go
- teleological modality: that which is necessary to achieve an aim as in, e.g.:
  If you want to get into university you need to study hard
  We must leave now or we'll miss the train
  This form of modality is often expressed with conditional structures as above.
Dynamic modality is often referred to as ability or personal modality (because that is what is mostly concerns). That is not compatible with the view taken in these sections of the site because we also consider expressions of willingness as falling into this category.
Alethic modality is sometimes referred to as circumstantial modality because it derives from the circumstances of the universe and expresses universal truths (which epistemic modality does not). For example:
Paper will spontaneously ignite at 233° C
which is true wherever you are and not dependent on when or where you state it.
For obscure reasons, you may also see this type of modality described as root modality but not on this site.

For teaching purposes and for the analysis of language for planning, these subtleties are probably unnecessary, although some languages deal with the categories differently, using a variety of means, often the subjunctive or other forms of verbs. However, if you are interested in the philosophy of language, they have some utility.
That said, for single lessons, it is often wise to take just one or two sub-sections of the main types of epistemic and deontic modality to keep the focus and help learners acquire the concepts. It also makes some sense to handle volition and ability separately when considering dynamic modality.

Modal (auxiliary) verbs

On many training courses, attention is given to what are, rather loosely called modal verbs or, even more loosely, just modals. The most obvious problem is that there is little consistency in the literature (or in teachers rooms) concerning what should be analysed as what.

All these verbs are considered on this site and can be tracked down from the general index of modality linked in the list at the end.

An alternative analysis of modality

The following is not meant to be exhaustive but, for most English-language teaching purposes, it will suffice as a starting point when planning to teach modality based on mental processes rather than form or vague semantic categories.
The alternative modal expressions are suggestive only and include modal adverbials, nouns and adjectives.

	Epistemic modality
That must be the teacher's desk

Modal auxiliary verb	Example	Alternative modal expressions
might	If there's too little current, the pump might not work He might have telephoned while I was out Might she have left early?	It's possible the pump isn't working. Possibly, he's already gone out. I'm assuming the pump isn't broken. My assumption is that that's the postman. I'm sure that is what he said. It's obviously his brother. It's clear they are related. It seems likely that it's him. I doubt she is there. I expect that's the postman
could	It could rain tomorrow He couldn't have gone out this early Could he already be here?
can	The pump can't be broken. It's new It can get cold in the winter Can it be her?
should	Ah, that should be the postman now It shouldn't be late Should the train come soon?
would	Well, he would, wouldn't he? It's in his interests That would be his mother you saw Would that be the right number?
must	That must be his brother. Aren't they alike?
may	Well it may be his brother. I don't know She may not be the right person to ask
will	That will be the postman at the door That won't be the postman. It's too early Will that be your mother ringing?
ought to	She ought to be there by now

On the right, there are examples of positive, negative and interrogative forms. Where one or more is missing, the forms are not used:

may in an interrogative always implies permission rather than likelihood
ought in the negative and interrogative always implies advice or prohibition
The negative and interrogative of must for epistemic modality is encoded with couldn't / can't

For more on how epistemic modality is expressed in English, see the guide to expressing (un)certainty and the fuller guide to epistemic modality, both linked from the list of related guides at the end.

	Deontic modality
They shall not pass

Modal auxiliary verb	Example	Alternative modal expressions
might	Might I ask a question?	Is there a chance of asking a question? Do you have a moment? Seeing a doctor would be advisable. I forbid you to speak to me like that. Is he allowed to come, too? Do you have permission to be here? I don't imagine this work will take long. They are advised to get here early. If I were you, I'd see a dentist. It's her duty to write. Registration is compulsory. I advise you to leave soon.
could	Could I just say something?
can	Can I see you for a minute? You can go You can't sleep here
should	You should not speak to me like that Should I wait for you? You should see a doctor
must	You must finish the work before 6 You mustn't turn right here Must I get a ticket?
need	This work needn't take long They need to get here early Do you need to see me?
may	May he come with us, please? You may not use a dictionary You may start
will	You will not speak to me so rudely again You will stay until we have finished
ought to	She ought to write to her father Ought you to go soon? I oughtn't to be so impatient with her
have (got) to	You have (got) to register online Have I (got) to explain again? They don't have to come if they don't want to
had better	Hadn't you better leave soon? I had better go soon You had better not try that again
shall	You shall not speak to your father like that Officers shall be properly dressed at all times

On the right, there are examples of positive, negative and interrogative forms. Where one or more is missing, the forms are not used:

might in positive statements and negatives usually refers to likelihood
could in a positive or negative form usually implies ability rather than permissibility
will and shall in an interrogatives usually refer to futurity or willingness

In some of the above, of course, there is an implied reference to possibility (i.e., epistemic modality). For example,
    Do you have a moment?
can be interpreted as a question about a current state but is more likely to mean
    Do you give me permission to interrupt your day?
and
    I think registration is compulsory
can be interpreted simply as a statement of opinion but is more likely to mean
    You probably have to register.

The verb need can also imply alethic modality as in, e.g.:
Dogs need walks
which implies a simple truth rather than an obligation per se.
The verb also has a non-modal use as a lexical or main verb meaning require as is, e.g.:
We are going to need a bigger boat

For more on how deontic modality is expressed in English, see the guide to expressing obligation and the fuller guide to deontic and alethic modality, both linked from the list of related guides at the end.

	Dynamic modality
We can do this!

Some analyses of dynamic modality include uses which are better seen as either deontic (obligation or its lack) or epistemic (to do with the truth of a proposition) so are excluded from this list.
Strictly speaking, dynamic modality is confined to expressions of ability or willingness. That will limit the number of modal auxiliary verbs quite severely to can, could, will, would and be able to for the most part (and the last of those is more sensibly analysed as a modal adjective).

Modal auxiliary verb	Example	Alternative modal expressions
can	I can speak Spanish well. Can you believe it! I can't manage alone	She has the ability to speak 4 languages. Is that believable? I haven't the strength to do it. I am happy to accept. Please help me. I volunteer to do that. Let me go; I know the way. Did you manage to find what you wanted?
could	I couldn't possible lift it She could read well at the age of 4 Could you give me some help?
will	I'll get the door I won't interfere Will you do the washing up?
would	I would love to come I wouldn't give you the money, anyway Would you come with me, please?
able to	Were you able to find what you wanted? I wasn't able to see her She was able to get it all done

For more on how dynamic modality is expressed in English, see the fuller guide linked from the list of related guides at the end.

	Alethic modality
The blue bit can't go here.

Modal auxiliary verb	Example	Alternative modal expressions
must	A quintet must contain 5 players.	Unless it has five players, it's not a quintet. Having more or fewer than 8 players disqualifies it as an octet. It is necessary to have 4 sides in a square.
can't	An octet can't have more or fewer than 8 players.
need	A square needs 4 sides.

When expressing alethic modality, the verb must is negated in standard English as can't or couldn't (as it is for epistemic modality). In some dialect forms, however, mustn't is used to express impossibility rather than negative obligation. So we can encounter:
    That mustn't be the right number
which for most speakers suggests deontic modality, instead of the standard:
    That can't be the right number
The verb can is not used in the positive for this meaning as it implies epistemic, deontic or dynamic modality in that form.
The verb need is somewhat rarer in this sense but does occur in, for example:
    The temperature needs to be over 40°C for this to work
It is not used in the negative in this sense.

It is also quite rare for alethic modality to be expressed in the past because, obviously, it usually refers to timeless facts which are independent of the speaker's existence. It is occasionally encountered when a fact has been superseded and is no longer true, for example:
    At that time a battle had to be fought on foot.
in such cases, the normal rule of replacing must with have to and can't with couldn't applies.
However, the verb need is not negated in the past when it is used to express alethic modality, so, e.g.:
    At that time armies needed to move solely on foot
cannot be negated as
    At that time armies didn't need to move solely on foot
or
    At that time armies needn't have moved solely on foot
because that implies a lack of obligation rather than a negative fact.

The guide to expressing degrees of likelihood, linked below, includes considerations of alethic modality and considers the overlap between it and epistemic modality.

Multiple modalities

It is possible, in all languages, to combine modalities in single utterances such as:
    She should be able to do that (combining epistemic and dynamic modalities)
    They may have to see a doctor (combining epistemic and deontic modalities)
    Peter might be allowed to go home early (combining epistemic and deontic modalities)
and so on.
The issue in English in particular is that the rules of the language's syntax do not allow the co-occurrence of pure or central modal auxiliary verbs so we cannot have, e.g.:
    *She must can come with us
as we may in many other languages.
There are a number of workarounds for this problem in English which are discussed in the guide to multiple modalities, linked below, and many concern the use of semi- and marginal-modal verbs as well as modal nouns, lexical or main verbs, adjectives and adverbs.

Entailment

This is a term dealt with in more detail in the guide on this site to pragmatics, linked below, and it follows on from ideas of multiple modalities.

Entailment concerns the understanding of truth and therefore falls into the realm of epistemic modality so, for example, if someone proposes:
John saw a kangaroo while he was on holiday in Australia
Then we know not only that the statement appears to be true but also that:

It is possible to see kangaroos in Australia (epistemic modality)
John must have been in Australia (epistemic modality)
John was certainly on holiday (epistemic modality)
Kangaroos are visible (alethic modality)
John must be able to see (epistemic and dynamic modality)
but not
John was certainly not in Australia
There are no kangaroos in Australia
John cannot see
Kangaroos must be invisible
John never has a holiday

The last five propositions are automatically disallowed by the proposition we began with and are not expressive of any form of modality but the first five are entailed in the comment.

In terms of modality, entailment is a useful concept because, for example:

I can help with that
is probably an example of dynamic modality expressing willingness, but it also entails another form of dynamic modality expressing ability. It may, in fact, be an example of an expression of ability but it is unlikely to be said if that, too, did not entail an expression of willingness (otherwise, why say it?). One of the rules of speech identified by Grice is that we do not normally over inform and if this comment does not entail an expression either of willingness or ability, it would break that maxim.
You should write to your mother soon
is clearly an example of deontic modality, expressing the speaker's view that an action is advisable but it also entails dynamic modality because it would not be said if the speaker did not believe the hearer was able to write to her mother.
God could make a kangaroo with three tails but he couldn't make a triangle with four sides
is clearly a comment concerning dynamic modality (positive and negative ability) but there is an entailed alethic modality embedded in the comment because we know that a triangle must have three and only three sides.
I'll buy some petrol so I can use the mower this afternoon
is a statement of commissive deontic modality with an obligation put by the speaker on the speaker but there is also an entailment of epistemic modality concerning the speaker's view that the mower will not function unless petrol is supplied and there is a further sense of dynamic modality concerning the speaker's expression of ability.

Modality is not simple.

Miscellaneous notes

In some analyses, the modal auxiliary verb must is classified as representing intrinsic or even (mistakenly) dynamic modality because it is seen as personal to the speaker rather than an externally imposed duty and have to appears under extrinsic deontic modality because, the theory has it, it expresses an externally imposed duty. However, where it is not obvious which is intended, native speakers use them in free variation.
It is also the case that it is difficult to separate the external from the internal so, for example:
I have a toothache and have to go to the dentist
can just as well be expressed as:
I have a toothache and must go to the dentist
because it is unclear whether a toothache represents an internal or external phenomenon.
The strength of both deontic and epistemic modality is often determined by intonation and stress rather than depending on the modal expression chosen by the speaker.
In this analysis, logical deduction and logical necessity are distinguished (as epistemic and alethic modalities) but, for most teaching purposes, can be conflated.
Teaching modal expressions using this kind of analysis presumes that you start from the type of modality your learners need to express and work on a limited range of ways to realise the mental processes in English.
There is a guide on this site, linked below, to teaching modality.

Summary

Here's a summary with examples using pure (or central) modal auxiliary verbs only. Semi-modal auxiliary verbs and marginal modal auxiliary verbs can also express different types of modality. All four types of modality can also be expressed without modal auxiliary verbs at all.
For teaching purposes, the summary is useful because you can see at a glance whether the lesson you planned mixes the same verb expressing different forms of modality. That can confuse, especially at lower levels.

examples and summary

Here's another summary which contains a few of the other sub-categories you may encounter along with some examples as an aide memoire.
For an explanation of them, go to the dedicated guides, linked below.

overall

And here is a final set of examples to show five ways in which the four types of modality are commonly achieved in English. It's not complete and there are more examples in the guides linked at the end. In particular, the guide to modality without modal auxiliary verbs has many more examples of other ways to realise modal ideas.

To check your understanding, try a multiple choice quiz on this area.

Related guides
the modality index	for links to all relevant sections
essential guide to modality	a simpler guide in the initial training section
modality map	a way to find your way around a complex area containing a clickable map
central modal auxiliary verbs	a traditional view taking each modal in turn and identifying its function
semi-modal auxiliary verbs	which also considers marginal modal auxiliary verbs such as seem, tend, be about to etc.
modality without modal auxiliary verbs	which considers adverbials, verbs, adjectives and nouns used to express modality
epistemic modality	modality for expressing the speaker's view of the truth or a proposition, i.e., likelihood
expressing uncertainty	a functional approach to epistemic modality
deontic modality	modality for expressing the speaker's view of obligation or its lack with some consideration of alethic modality with which it overlaps
dynamic modality	modality for expressing ability and willingness
multiple modalities	for the guide to how English manages to combine types of modality
pragmatics	for more about entailment and much else
complex tenses	which also considers complex tenses in relation to modal auxiliary verbs
teaching modality	for some more ideas transferable to the analysis above
modality and aspect	which considers modal auxiliary verbs with perfect and progressive forms and considers some of the types of modality discussed here
test yourself	go here for a set of six linked tests on types and realisations of modality

Reference:
Palmer, FR, 2001, Mood and Modality, 2nd ed., Cambridge: Cambridge University Press
and some more.