A relative clause is a subordinate clause (a clause that can’t stand by itself as a complete sentence) that describes or modifies a noun. Notice how the relative clause in each of the following sentences (shown in italics) describes the nown that it follows:
This is the computer that I want to buy.
The muscles that hurt the most are in my shoulders.
The priest, who was from Canada, met us at the church gate.
These two subjects, which are closely related, are the really important ones.
There are two kinds of relative clauses: defining (or restrictive) and nondefining (or nonrestrictive). A sentence with a defining clause needs the clause in order to make sense; a sentence with a nondefining clause doesn’t actually need the clause in order to make sense. The first two of the four sentences above have defining clauses, and the last two have nondefining clauses. The first two sentences below are grammatically correct but don’t contain all the information that they need:
This is the computer.
The muscles are in my shoulders.
The priest met us at the church gate.
These two subjects are the really important ones.
A relative clause generally begins with a relative pronoun, usually that, which, who, or whom. Who and whom only refer to people (and sometimes animals); that refers to things and often to people and animals; which refers to things but never to people. Nondefining clauses never begin with that.
Notice that clauses can be identified by their punctuation. A nondefining clause is separated from the rest of the sentences by commas; a defining clause is not.
My house, which I bought in 2002, is now worth $250,000.
(Not: My house that I bought in 2002 is ow worth $250,000)
Thieves stole her car, which she parked in the wrong place.
(Not: Thieves stole her car that she had parked in the wrong place)
Notice also that that may only introduce defining clauses, but which may introduce both defining and nondefining clauses.
This is the computer which I want to buy.
That is often used in place of who or whom, but which never is.
Those are the girls that (or who or whom) I want you to meet.
The doctor that (or who or whom) we like best is Dr Hall.
When a relative pronoun is the object of a defining clause, it can be omitted:
That’s the man (who or whom or that) I saw yesterday.
The fruit (that or which) they served was extremely odd.
She was the actress (who or whom or that) the playwright wrote the part for.
But the pronoun can never be omitted when it is the clause’s subject:
There’s the man who caught the thief.
(Not: There’s the man caught the thief)