Grammar study page: relative clauses

Defining relative clauses are used to specify which person or thing we mean. We don't put commas between the noun and a defining relative clause.
Who or that are used for people.
Which or that are used for things.

I have a friend who speaks five languages.
I have a friend that speaks five languages.

She showed me the coat which she had bought.
She showed me the coat that she had bought.

Non-defining relative clauses (extra information clauses) are used to add extra information which is not necessary. We put commas between the noun and a non-defining relative clause.
Who is used for people.
Which is used for things.
That cannot be used.

Mr Fry, who speaks five languages, works as a translator for the EU.

The area, which has very high unemployment, is in the north of the country.

We use who or that when we talk about people.

Who is more formal than that.

This is the man who helped us.
(more formal)

This is the man that helped us.
(less formal)

We use which or that when we talk about things (not people).

Which is more formal than that.

It's the watch which my husband bought me for my birthday. (more formal)

It's the watch that my husband bought me for my birthday. (less formal)

Which and that can be left out of a defining relative clause when the pronoun refers to the object of the sentence.

It's the watch my husband bought me for my birthday.

In this sentence, 'the watch' is the object of the verb 'bought' and so we don't need to use a relative pronoun like that or which.

Which and that cannot be left out of a defining relative clause when the pronoun refers to the subject of the sentence.

It was the man that sold me the car.

In this sentence, 'the man' is the subject of the verb 'sold' and so we need to use a relative pronoun such as that or who.

We use whose to show possession.

John, whose brother was also a musician, plays over 100 concerts every year.

We use where when we talk about place.

My wife and I went to the bar where we first met.

The man who was sitting next to me had a brown jacket.

After a relative clause we do not repeat the subject.

The book that I bought was by Edward Lear.

Only that or which are possible, not what.

He lent me the book, which I found very useful.

A relative clause can have only one direct object.

The winner, who was 25, will receive $12,000.

That cannot be used in a non-defining (extra information) clause.

Our office, which I share with my two colleagues, is about two kilometres from the city centre.

A relative clause follows the noun to which it refers.