A relative clause is a type of subordinate clause that provides additional information about a noun in the main clause. It functions as an adjective, adding details to the noun it modifies. Relative clauses are introduced by relative pronouns (such as who, whom, whose, which, or that) or relative adverbs (such as where, when, or why).
Relative clauses give us information about the person or thing mentioned.
Defining relative clauses give us essential information – information that tells us who or what we are talking about.
The woman who lives next door works in a bank.
These are the flights that have been cancelled.
We usually use a relative pronoun or adverb to start a defining relative clause: who, which, that, when, where or whose.
We can use who or that to talk about people. that is more common and a bit more informal.
She’s the woman who cuts my hair.
He’s the man that I met at the conference.
We can use which or that to talk about things. that is more common and a bit more informal.
There was a one-year guarantee which came with the TV.
The laptop that I bought last week has started making a strange noise!
when can refer to a time.
Summer is the season when I’m happiest.
where can refer to a place.
That’s the stadium where Real Madrid play.
whose refers to the person that something belongs to.
He’s a musician whose albums have sold millions.
Omitting the relative pronoun
Sometimes we can leave out the relative pronoun. For example, we can usually leave out who, which or that if it is followed by a subject.
The assistant [that] we met was really kind.
(we = subject, can omit that)
We can’t usually leave it out if it is followed by a verb.
The assistant that helped us was really kind.
(helped = verb, can’t omit that)