Relative pronoun clauses

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Definitions The dog which howled all night

Here's a definition from Parrott who avers that relative clauses are:

complex structures which allow the speaker to express themselves succinctly and fluently
(Parrott, 2000, p. 381)

They actually do rather more than that but it's a good working definition to begin with.

Relative pronoun clauses are usually said to be clauses starting with who(m), that, which, whose defining or identifying the noun they follow.
Their function is one of subordination to which there is a guide on this site.

So, for example, in
    The dog which howled all night and kept me awake
The noun, dog, is rendered unique among millions of dogs because only this one howled and caused a sleepless night.
In this sentence, the relative clause, which howled all night and kept me awake, is acting to modify the noun, dog, and that makes it adjectival in function.  For this reason, some analyses will refer to these structures as post-nominal adjectival modifiers (Celce-Murcia & Larsen-Freeman, 1999, p 571) or adjective clauses (Yule, 1998, p 240).
We'll stick with relative pronoun clause here because it is more familiar but those two descriptors are equally valid.
The key point is that relative pronoun clauses modify and/or identify an already specified noun phrase and in that sense they are akin to adjectival phrases.

Some languages, see below, rely on using adjectival expressions alone to do this task, having nothing remotely like the English relative pronoun structures.  For speakers from these language backgrounds, the concepts and meaning are, initially, at least, obscure.

If you are wondering why where, when and how are not in the list above, the answer is that these words are analysed elsewhere in the guide to relative adverbs.
Relative pronouns, as the name implies, are words which stand for a noun, a gerund, a noun phrase or a nominalised clause (i.e., different sorts of nouns).
If the word refers to why, where, when or how an action is carried out or a state exists, it is an adverb, not a pronoun and does not belong here.
That relative pronouns and relative adverbs appear to be related is not in question but relative adverbs function differently and perform different grammatical tasks.  Relative adverbs cannot, for example, appear as the objects or subjects of verbs (because they aren't nouns of any sort).  One relative adverb, how, cannot refer to a noun at all.
As we shall shortly see, relative pronouns are distinguished by the functions they perform in sentences, technically their relationship to the arguments (most commonly subject, direct object, indirect object etc.).
Understanding how to use them relies on being able to untangle the grammatical functions of phrases and clauses in sentences.

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Defining vs. Non-defining Restrictive vs. Non-restrictive Identifying vs. Non-identifying

Again, the terminology varies.  Here, we will use defining and non-defining because the terms are the most familiar but the three pairs of ways to describe the fundamental types are synonymous.
Here are four sentences to compare:

• The Statue of Liberty, which stands in New York, is well known.
• The Statue of Liberty which stands in New York is well known.
• The statue of Gandhi which stands in Tavistock Square is well known.
• The statue of Gandhi, which stands in Tavistock Square, is well known.

Before you go on, decide which of those sentences you are happy to accept.

What's the difference in meaning between these pairs of sentences?  Click here when you have an answer.

1. At the first meeting, which was held yesterday, the chair invited comments from everyone.
2. At the first meeting which was held yesterday the chair invited comments from everyone.
3. The kids, who came with me, had lunch on the train.
4. The kids who came with me had lunch on the train.

Now look again at the first four sentences concerning statues and decide what the issue was.  Here's the comment:

Sentence	Issue
The Statue of Liberty, which stands in New York, is well known.	The clause between the commas is simply adding information concerning the location of something known to us all.
The Statue of Liberty which stands in New York is well known.	There is only one such statue so to omit the commas would be wrong.  You cannot define that which is unique.
The statue of Gandhi which stands in Tavistock Square is well known.	There are many statues of Gandhi around the world so to define a particular one by where it is is acceptable.
The statue of Gandhi, which stands in Tavistock Square, is well known.	This is also acceptable.  Here we are talking about a statue of Gandhi but adding information to say where it is, not defining it.

Try another slightly different example:
In the following, why is it not possible to take out the commas in the first sentence or to insert them in the second?  Click when you have an answer.

5. The Nile, which runs through Egypt to the Mediterranean, is vital to the country’s prosperity.
6. The man who asked me to marry him on that memorable evening is still my husband.

Defining (restrictive / identifying) relative pronoun clauses are by far the most common.

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Meaning

The difference between restrictive / defining and non-restrictive / non-defining relative clauses is not just a grammatical wrinkle in English because the meaning the speaker / writer conveys differs very significantly depending on which type of clause is used.
The key point is that defining clauses give required information whereas non-defining clauses give additional information.
The distinction is a familiar one concerning given and new information.

For example, in these two sentences, we have a clear distinction in meaning:

1. The woman, who arrived at the hotel this evening, has gone out
2. The woman who arrived at the hotel this evening has gone out

In sentence a., both speaker and hearer are aware of the existence of the woman and know to whom reference is being made.  The information about the hotel is new.
In sentence b., this is not the case and the speaker selects the grammar accordingly because the necessary information to distinguish which woman is being spoken of is needed for comprehension.  The assumption in sentence b. is that the hearer already knows about the woman and that she arrived at the hotel.  The only new information is that she has gone out.
This is even clearer when we use names rather than common nouns.  Compare, for example:

3. Margaret, who arrived at the hotel this evening, has gone out
4. Margaret who arrived at the hotel this evening has gone out

In sentence c., the person referred to is known to both speaker and hearer and the information about the hotel is peripheral but probably new to the hearer (why else state it?).
Sentence d. is very strange and it is difficult to imagine it ever being produced unless there are two Margarets to whom reference may be being made and the speaker needs to disambiguate to refer only to one of them.  In this case, the speaker assumes that the hearer knows there are two possible people called Margaret to whom reference is being made and that one of them arrived at the hotel and one did not.

One way to explain this distinction to learners whose languages may not make any distinction at all between the two types of clause is to consider a simple attributive adjective phrase in a sentence such as:
    The hardworking students passed the examination
which is ambiguous because this may mean either:
    The students who worked hard passed the examination (and other students, less hardworking, did not)
or
    The students, who worked hard, passed the examination (all of them)

It is difficult to signal what is meant by intonation n the adjective phrase, although stressing the adjective strongly might show that a restrictive meaning is intended.  In the written form, no punctuation changes can be made to show what is meant, although one could resort to underlining or other emphasis markers.
The two relative pronoun clauses are much more easily distinguished by intonation and punctuation signals.  See below.

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Next question

If you use phrases like all of which, either of whom, both of which, the majority of whom, none of which, etc., would you normally expect to separate the clause off with commas or not?  Click here when you have an answer.

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Pronunciation

In written English, commas are used to distinguish the two types.  How does this work in spoken English?  Click here when you have an answer.

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Memory test

There are five relative pronouns in English, listed at the top of this page – what were they?
Click here when you have written them down.

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Subject or Object?

It's important to know whether the relative pronoun is acting as the subject or the object of the verb.  What's it doing in the following examples?  Look at the underlined clauses and decide if they refer to the subject or object of the verb phrase.
Click when you have an answer.

7. The man who bought the tickets really is just being generous
8. The tickets, which hopefully will allow us entry, are very welcome
9. The man that we thanked seemed genuinely surprised
10. The tickets which he bought were quite expensive
11. Only the senior doorman, who we gave the tickets to, noticed that they were fakes

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Omitting the pronoun

In which of sentences 7 – 11 can you omit the relative pronoun?
Why (not)?
Here they are again:

7. The man who bought the tickets really is just being generous
8. The tickets, which hopefully will allow us entry, are very welcome
9. The man that we thanked seemed genuinely surprised
10. The tickets which he bought were quite expensive
11. Only the senior doorman, who we gave the tickets to, noticed that they were fakes

Click here when you have an answer.

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	Using that
The man that she met

Here are the example sentences again with some more at the end:

1. At the first meeting, which was held yesterday, the chair invited comments from everyone.
2. At the first meeting which was held yesterday the chair invited comments from everyone.
3. The kids, who came with me, had lunch on the train.
4. The kids who came with me had lunch on the train.
5. The Nile, which runs through Egypt to the Mediterranean, is vital to the country’s prosperity.
6. The man who asked me to marry him on that memorable evening is still my husband.
7. The man who bought the tickets really is just being generous
8. The tickets, which hopefully will allow us entry, are very welcome
9. The man that we thanked seemed genuinely surprised
10. The tickets which he bought were quite expensive
11. Only the senior doorman, who we gave the tickets to, noticed that they were fakes
12. The man whom we met turned out to be his brother.
13. The man who met us was his brother.
14. The table which I wanted had been sold.
15. The table which cost too much was the only one left.

In which of these sentences 1 – 15 can that be used as the relative pronoun instead of who(m) or which?  What's the rule?
Click here when you have an answer.

By the way, there is a structure in English using that which looks a bit like a relative clause but isn't and it will confuse learners if you introduce it alongside proper relative pronoun clauses.  It is, for example:
    It was because I felt ill that I had to go to bed
The word that is not, in this case a pronoun at all, arguably, because there is no noun for it to represent.
It is better analysed as a cleft sentence, to which there is a guide on this site linked in the list of related guides at the end.

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Prepositions in relative clauses

The following sentences contain prepositions.  How are these significant?
What is the rule for dealing with prepositions in relatives?  Click here when you have an answer.

16. This is the car in which he arrived.
17. This is the car which he arrived in.
18. This is the car he arrived in.
19. This is the person with whom he arrived.
20. This is the person who(m) he arrived with.
21. This is the person he arrived with.

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Reduced relative clauses

Consider these four sentences:

22. The woman in the garden is my mother.
23. The woman outside is my mother
24. The kid acting the fool is my sister.
25. The woman, an expert on gardening, is helping my mother

When is it possible to omit both the relative pronoun and the verb be?  Click here when you have an answer.

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Nominal or fused relative clauses

In the analysis above, all the examples contain both the relative pronoun and what is known as its antecedent (i.e., the noun the relative pronoun refers to).  So, for example, in
    That's the man who stole my bicycle
it is clear that the man and who refer to the same person.  So, the man is the antecedent of the pronoun who.
and in
    The tickets which we sold to my brother
the antecedent of the pronoun which is the tickets.

Frequently, however, the antecedent is either understood or simply absent.  This is why clauses of this type are sometimes called fused relative clauses (because the antecedent and the pronoun are combined) or nominal relative clauses (because the whole clause is acting as a noun).
Here are some examples:

• I don't know who sold him the tickets
  in which who sold him the tickets is a nominal clause acting as the direct (and only) object of the verb know
• Whoever broke the window will have to pay for it
  in which the nominalised clause Whoever broke the window is acting as the subject of the verb phrase will have to pay
• We are arguing about who to invite to the party
  in which the nominalised clause who to invite to the party is acting as the complement of the preposition about
• This is what he wants you to do.
  in which the nominalised clause what he wants you to do is the subject complement of the copular verb be
• He gave whoever asked the money they needed
  in which the nominal clause whoever asked is acting as the indirect object of the verb give

As you see, nominalised or fused relative clauses fill the same grammatical slots as noun phrases (hence the name).

With nominal relative clauses certain relative pronouns are used:

1. what is the most common
   1. She told him what he wanted to hear (object)
   2. What I wanted had disappeared (subject)
   3. Have you any idea what to do next? (object of the interrogative)
2. who and which are also commonly used as object nominalised clauses
   1. I explained who was going to do the work
   2. I didn't know which to use
3. the -ever series of pronouns are also frequent
   1. Whoever told you that is a liar
   2. Whatever you want is OK with me
   3. Whichever you decide on will be expensive

Warning: some sources will include formulations such as
    How(ever) you do it is your business
    Whenever he comes is OK with me
but these are, in fact, relative adverbs, not relative pronouns (and the subject of a different guide linked in the list of related guides at the end).
If the word refers to when, how or why, it is not a relative pronoun, it's a relative adverb and they function significantly differently.  To mix them up, and hence confuse your learners, is unwise.
Some wh- and that-clauses occur in what are known as cleft sentences and there is guide to these on this site linked in the list of related guides at the end.

Unlike other nominal clauses, nominal relative clauses can act as the indirect object so we allow, for example:
    I'll give whoever asks a new book.

There are also structures called free relative clauses and those, too, are considered in the guide to relative adverbs because they share some characteristics.  As you will see, if you go to that guide, this site takes the view that nominalisation is a better way to think about such things.

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The position of the relative clause

As saw in many of the examples in the guide so far, relative pronoun clauses can occur in mid-position in a sentence, with or without the object pronoun.  For example:
    The man (whom) you met was my boss
    The book (which / that) you took was mine.
    The person who sold me the car has left the country
etc.

Relative pronoun clauses are also frequently found in the final position in sentences because they typically introduce new information (and that is a common phenomenon in English called end focus).
The antecedent is commonly an indefinite pronoun such as one of the any- or some- series or an indefinite noun phrase such as a person, a child, a thing, a tool etc.  For example:
    I’m looking for someone who can help me write a website
    We need something which can act as a counterbalance
    Can you find us a local who can show us the way?
    I don't have the tool that will do the job
    Is that the man whose father is an MP?
etc.
This position is common with an existential there is / there are structure.  For example:
    Is there anyone here who can help me?
    There is something over there that looks like a snake
    There are some things (that) I always forget to pack
etc.
By the nature of such sentences, the relative pronoun is often the subject and cannot, therefore, be omitted.

Non-defining relative clauses almost always occur in mid-position so we can have:
    My sister, who lives in America, may be able to help you with that
but not:
    *My sister may be able to help you with that, who lives in America

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Stacking pronoun relative clauses

Relative pronouns clauses can also be stacked in the final position so we can have, for example:
    That is the book I got from the library which I like but that you hate
In theory, there is no limit to how many relative pronoun clauses can be stacked in this way but native speakers stop at two or three.

This is less common in mid-position because of the cognitive overload produced by trying say (and understand) something like:
    Have you given the food which I cooked and that you hated but which the guests enjoyed to the dog?

Non-defining clauses, probably up to a maximum of two, can be stacked in mid-position.  For example:
    His brother, who lives in France and who speaks French, may be able to translate that.
It is possible to have more than two of these, providing they are short enough so, e.g.:
    His brother, who lives in France, speaks French, can be contacted by email and is usually helpful, may be able to translate that
is possible but that's about the limit of the cognitive load with which speakers and listeners can cope.

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	Sentence relative clauses
... which shocked me

There is an odd form of relative clause in which it is not possible to identify an antecedent noun phrase because the reference is not to a particular person or object but to the whole of a preceding sentence (or even a longer text).  The name for this varies in the literature but here we will refer to them as sentence relative clauses although you may find them called comment clauses and a number of other things.
The only one of the five pronouns which can function this way is which.
Here are some examples:

    They fell in love and got married, which astonished everyone they knew.
    After the rain, the garden flourished, which was no surprise.
    Once we had had the meeting, matters improved, which was welcome.

These clauses are generally separated from the antecedent text by commas because they are, in fact, non-defining, in the sense that the preceding text can sensibly stand alone.
They always follow the text to which they refer.
It is not possible to omit the pronoun.

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Other languages

Relative clauses in English come after the noun to which they refer.  The reason for this has to do with how elements are ordered in English in terms of what is known as heaviness.
A heavy element of a clause is more grammatically and lexically complex than a light one.  For example, modifying a noun with a determiner or simple adjective as in:
    that car
    a car
    the car
    three cars
    red car
is a very light way to do so and English prefers light elements to precede the noun so we do not get
    car that
    cars a
    car the
    cars three
    car red
and so on.
This is not the case, incidentally, in a range of other languages, including Yoruba and many African languages as well as Thai and many other South-East Asian languages.
However, modification with a complex prepositional phrase will normally follow the noun because it is a heavier element than a single determiner or adjective so we have, e.g.:
    the car over there with the broken headlight
not
    the over there with the broken headlight car.
Relative clauses are just about the most complex way in which noun phrases can be modified in English so it is unsurprising that the clauses follow the noun they modify or refer to.  That is not a universal tendency of all languages but it is one shared with most European languages.

The ways in which other languages form relative clauses is very varied and first-language interference errors are common when learners are acquiring the system in English.
Here's brief run-down of the most important differences.  It behoves you to find out how your learners' first language(s) handle the area so you are prepared for the interference issues and can focus on salient differences.  If you are a native or very competent speaker of your learners' first language(s), this just requires a little thought.  In other circumstances, it requires a little research.

1. who vs. which
   Many languages, including but not limited to:
   Dutch, Albanian, Scandinavian languages (including Finnish, this time, and Icelandic), German (in which relative pronouns closely parallel the form of the definite article), Spanish, French, Italian, Malay / Indonesian, Latvian, Maltese, Portuguese, Greek, Russian, Polish (and other Slavic languages including Slovak, Czech and Slovene etc.), Farsi and Thai
   do not distinguish between a pronoun referring to people and one referring to inanimate objects and animals.  In most languages, therefore, there is no pronoun distinction between
       The wind which came in
   and
       The man who came in
   so errors such as
       *That's the man which I saw
       *That's the table who he bought in France
   etc. are frequent and have to be handled by making sure that the distinctions in English between who and which are very clear.
   Speakers of these languages, aware that English is different in this respect, may be tempted to play safe and overuse that in grammatical environments where it is not allowable (i.e., in non-defining or non-restricted clauses).  Errors such as:
       *The Eiger, that is in The Alps, is a beautiful mountain
   or
       *The bus, that I take every morning, was very late today
   will occur.
   Additionally, that is often unacceptably informal, especially in writing, and that leads to stylistic error.
   Finally, some of these languages use that and what interchangeably and that produces errors like
       *That's the man what he said would come
       *The wind what came in
       *The man what came to the meeting
   etc.
2. Non-parallel structures
   Even languages which sometimes distinguish between pronouns for people and things do so in ways which do not parallel English at all.
   In French, for example, different pronouns are used depending on whether the object or the subject is the reference (que and qui respectively) but both can be used for animate and inanimate nouns.
   Italian uses different pronouns depending on whether the noun is followed by a preposition or not (cui and che respectively).
   Portuguese has a single relative pronoun (que) which can refer to people or things but this changes to quem when preceded by a preposition.  The word quem is only used to refer to people and means who or whom.
   All of these differences can lead to interlingual errors because of temptation to assume that differences like these are paralleled in English.
   A further non-parallel aspect is the use of which as a sentence relative clause to refer to a preceding text rather than an identifiable antecedent noun phrase.  Most languages will not use a pronoun in this way, preferring something like ... and that + the comment or using an equivalent of what.
3. Omitting the pronoun
   Most languages which use relative pronouns as subordinators do not allow the pronoun to be omitted and, apart from causing speakers to sound unnaturally formal with sentences such as
       The book which I read explains it well
   instead of the much more natural
       The book I read explains it well
   this also presents comprehension difficulties because learners from these backgrounds will have problems understanding
       Is she the woman you spoke to?
   or
       That's the program you should run
   because there are no obvious pronoun clues to what the object of the verb really is.
   Languages which do not allow the pronoun omission are in the majority and include most of those listed in point 1, above.
   The omission of the pronoun in English, incidentally, causes serious problems for machine translations for the same reason that comprehension issues arise.
4. Restricted and non-restricted uses
   A range of languages, including Russian, German, Dutch and Polish, do not distinguish between restricted (defining) clauses and non-restricted (non-defining) clauses and that can cause punctuation, pronunciation and comprehension errors.  In German, for example, all relative clauses are separated by commas, not just non-defining clauses as in English.
   For speakers of other languages, in which the comma is used to separate sense groups rather than represent pausing, similar issues arise.
5. Concord
   Most languages (including American varieties of English) are strict about concord and will strive to make verb and pronoun forms match the number and characteristics of the subject.  English is sloppier in this respect and, depending on their notion about the nature of the subject, English speakers can accept, for example:
       The group who were asked to work on the project
   and
       The group which was asked to work on the project
   but will not allow
       *The group which were asked to work on the project
   or
       *The group who was asked to work on the project
   This will lead to some error because most learners will assume that English has the same grammatical regard for concord that their languages exhibit.
   Other concord errors will occur, such as:
       *The range of students which was accepted for a place at the university
   which follows a grammatical rule (range is non-animate and singular and so should be followed by which and a singular verb form) but is unacceptable to most English speakers who would prefer:
       The range of students who were accepted for a place at the university
   These are examples of issues with notional and proximity concord respectively in English.  For more on concord, follow the link below.
6. Absence of relative pronouns and clauses
   You will search in vain for mention of relative pronouns in the grammars of many languages, including Turkish (usually), Korean, Tamil and Japanese, for the simple reason that these languages do not use them at all and no closely equivalent structures exist.
   In most, the meaning is expressed either through compound adjectives, so we get:
       *The by the river house
   instead of
       The house (which is) by the river
   or by participle clauses so we get, e.g.:
       *My friend living in America invited me to visit him
   instead of
       My friend who lives in America invited me to visit him
   Some of these language, Turkish being an obvious example, do have a structure similar to a relative clause but the language uses non-finite forms rather than finite ones which English prefers.  So, for example, a Turkish speaker with have little difficulty understanding or producing:
       The car sitting in front of the house is his
   but will not naturally produce and may have difficulty understanding
       The car which broke down is in front of the house
   and may attempt something like:
       *The car breaking down is in front of the house
7. Resisting subordination
   South Asian languages in particular resist subordination altogether, preferring other ways to express the fact that one part of a sentence depends on the understanding of another, and speakers of these languages (which include many in the subcontinent such as Hindi and Urdu) may have difficulty seeing the reason for relative clauses at all and struggle with both the forms and the whole concept of subordination in general.
8. Clause ordering
   Finally, in Chinese languages, there are parallel structures but in these languages, the relative clause precedes rather than follows the noun phrase and that can produce errors such as:
       Who is in England my friend wrote to me
   Additionally, the pronoun may routinely be omitted regardless of the grammatical function it performs and that leads to errors such
       *The man gave me the money was very friendly
   This will also affect languages which use adjectival phrases or participle clauses to express the concepts (see point 6) because these structures also precede the noun and they may produce errors such as
       *The parking the car woman ...
   instead of
       The woman (who is) parking the car
   or
       *The book buying the man is very expensive
   instead of
       The book (which) the man is buying / bought is very expensive
   The distinction between the times when one can and cannot omit the pronoun and the ordering of the clauses need careful work.

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Teaching relative pronoun clauses

If you have been following up to now, you will know that this is not a simple system in English and the restrictions, as we saw above, are important.  In particular, we (and, eventually, our learners) need to be aware of:

• when the pronoun can be omitted
• when the pronoun that can be used
• where the clause comes in the sentence
• which pronoun to select
• when the clauses can be reduced or nominalised

And these are just some of the issues you need to know about and analyse before you can begin to tackle the area.
For this reason, careful selection of model texts or other presentation texts is very important.
If the text you select contains multiple examples of relative pronouns as subjects, as objects, including that, reduced, with omitted pronouns, in nominalised clauses and so on you are asking for trouble.  This will be compounded if you also fail to distinguish between relative pronouns and relative adverbs.  For examples of the horrible, confusing mishmash that doing so produces, search the web for lessons in the area.  Alternatively, pick up any of a range of classroom coursebooks whose authors have neglected to do the research.

At lower levels in particular, you need to focus very carefully, introducing, say, only defining relative clauses using who and which as subject pronouns in model sentences such as
    He is the student who asks lots of questions
or
    That is the car which sat outside my house all night
Getting lower-level learners to form such sentences from:
    He is the student
    He asks lots of questions
or
    That is the car
    It sat outside my house all night
is a good beginning but it can't stop there, of course.

The window: a neglected classroom aid

A quickly set up exercise to practise relative pronoun clauses is to get your learners to look out of the classroom window and tell you what they see.  With a bit of priming and nudging from you, that can elicit comments such as:
    I can see a car which is parked illegally
    There is a traffic warden who is putting a ticket on it.
    The people whose car it is will be really upset
etc.
Clearly, this can't be done until the forms have been presented and practised a little but it works well.

The learners: another neglected classroom aid

Using your learners' own experiences (their internal world) can also be productive and it is not hard to set up practice routines to evince statements like:
    We went on holiday to New York last year which I found fascinating.
    We have a room in the house which we only use for guests.
    I remember a great teacher who really inspired his students.
etc.
It's up to you to limit and elicit the correct pronouns.

A semi-controlled practice idea is simple to get the learners to say something true about three other people in the room (including you, if you like) and three things they know about.  That can evince statements such as:
    Paul is the guy who always knows the right answer.
    Mary is the person who sits near the door
    This is the coat Mary wore this morning
    That's the pen Arthur stole
and so on.
It's a personalised and engaging exercise and, so the theory goes, makes the structures memorable.

The students' mementos: another neglected classroom aid

An associated idea is ask the learners to bring in half a dozen favourite photographs or souvenirs and get them to explain what they are of or where they came from. That can evince language such as
    It's a bag I bought in Tunisia
    It's a photograph I took in Russia
    The girl in the blue hat is my sister
and so on.
This works particularly well for practising the omission of the pronoun because, by their nature, the items are generally the objects of defining relative clauses.

The next step is to introduce the range (excluding that for which special rules apply) until the learners can construct acceptable subordinating clauses using which, who(m) and whose.
Then it's time to focus on when the pronouns can be omitted (restricted to defining clauses where the object is denoted by the pronoun, and excluding whose).

Finally, the range of reduced clauses can be tackled along with nominalised or fused relative clauses.  That can be done alongside consideration of wh- question forms because the structures are more or less parallel:
    I don't know what to wear
is not far removed grammatically from:
    Do you know what to wear?
and
    Do you know who arrived late?
because both have a nominalised wh-clauses as the object of the verbs.

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Summary

Here's a brief cut-out-and-keep summary of the area:

 

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Related guides
relative adverbs	for more on another form of relative clause using adverbs
wh- questions	for an area allied to the use of nominalised relative clauses
cleft sentences	for more on how wh- clauses are used in English
concord	for more on notional and proximity concord
subordination	for more on other ways to make ideas depend on others

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Click for a test in this area.

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References:
Campbell, GL, 1995, Concise Compendium of the World's Languages, London: Routledge
Celce-Murcia, M, Larsen-Freeman, D, 1999, The Grammar Book – An ESL/EFL Teacher’s Course 2nd ed., Boston: Heinle
Dryer, MS and Haspelmath, M. (eds.), 2013, The World Atlas of Language Structures Online, Leipzig: Max Planck Institute for Evolutionary Anthropology
Parrott, M, 2000, Grammar for English Language Teachers, Cambridge: Cambridge University Press
Swan, M and Smith, B, 2001, Learner English, 2nd Edition, Cambridge: Cambridge University Press
Yule, G, 1998, Explaining English Grammar, Oxford: Oxford University Press