Relative clauses beginning with question words (who(m), which, where, why when, whose) are often used to modify nouns and some pronouns, to identify people and things or to give more information about them. Clauses used like this are called relative clauses.
Do you know the people who live next door?
There’s a program tonight which you might like.
He lives in a village where there are no shops.
When "who", "whom" and "which" introduce relative clauses they are called relative pronouns. "Who(m)" refers to people and which to things.
What’s the name of the tall man who just came in?
It’s a book which will interest children of all ages.
"Who" and "which" can be the subjects of verbs in relative clauses.
I like people who smile a lot.
This is the key which opens the garage.
Whom and which can also be the objects of verbs in relative clauses. "Whom" is unusual in informal register.
Do you remember the people who we met in Romania? ("who" is the object of "met")
I forget most of the films which I see. (which is the object of see)
If the relative pronoun is the object of the clause, like those above, it is not necessary and can be left out. Actually, it’s quite unusual to keep them in. However, if the relative pronoun is the subject of the clause, it can NEVER be left out.
People often use "that" instead of "who" or "which", especially in an informal style.
I like people that smile a lot.
This is the key that opens the garage.
However, it can not be used in non-defining clauses, in which you can only use "who(m)" or "which".
All that, only ... that etc.
"That" is use especially after quantifiers like "all", "every(thing)", "some(thing)", "any(thing)", "no(thing)", "none", "little", "few", "much", "only", and after superlatives.
Is this all that’s left?
Have you got anything that belongs to me?
I hope the little that I’ve done has been useful./br> All that you say is certainly true.
Object pronouns can often be left out. Note: This is not possible in all relative clauses.
Do you remember the people we met in Italy?
As subjects or objects "who(m)", "which" and "that" replace words like "she", "him" or "it": one subject or object is enough.
He got a new girlfriend. She works in a supermarket.
He got a new girlfriend who works in a supermarket.
"Whose" is a possessive relative pronoun, used as a determiner before nouns. It replaces "his/her/its".
I saw a girl whose hair come down to her waist.
The car whose tires are flat is mine.
"Which" can refer not only to a noun, but also to the whole of previous clause.
Note that, "what" can not be used in this way, and also that there must ALWAYS be a comma before "which" in this kind of sentences.
He got married again a year later, which surprised everybody.
"When" and "where" can introduce relative clauses after nouns referring to time and places, while why refers to the reason.
I’ll never forget the day when I first met you.
On holiday, we saw the house where Shakespeare was born.
I don’t know the reason why she left me.
However, the last sentence sounds a bit strange although it’s perfectly correct grammatically. Most native speakers would use either of these two structures instead:
I don’t know the reason she left me.
I don’t know why she left me.
Relative clauses are divided into two categories: restrictive clauses and non-restrictive clauses.
Note: They are also called defining and non-defining clauses or identifying and non-identifying clauses.
The type of relative clause determines if a comma is required or not.
A restrictive clause explains which people, places, or things; that means not everything and not everyone.
My sister who lives in Holland is very rich. (I have 3 sisters but not all are rich. Only my sister who lives in Holland is rich. The sentence can be written with comma if I would have only one sister)
Many of the people who live in Vietnam are not very tall. (There are 86 million people in Vietnam, some are tall and some are short)
In the examples above the restrictive clause identifies which sister and which people ... (Not all of my sisters and not all Vietnamese people ...)
A non-restrictive clause just adds more information about the noun/pronoun. Clauses that modify proper names, entire groups, nouns that are unique (sun, moon etc) etc are usually non-restrictive.
The “Big C” supermarket, which is the cheapest in Saigon, is always crowded. (There is only one Big C supermarket in Saigon and the extra information is “which is the cheapest ...”; in this situation the non-restrictive clause just adds an extra information about the Big C) – Comma is required.
My motorcycle, which is a Yamaha, was very cheap. (In spoken English, short pauses are used before and after the clause)