In English we use defining relative clauses to give important information about a person, place or thing.

We use :

"who" or "that" for people

• He's the man who lives next door.
• She's the woman that works in the cafe.
• People who smoke often develop breathing problems.

"which" or "that" for things

• A clock is a thing which tells the time.
• This is the letter that I was talking about.
• This is the dress that I bought for $5.
• The novel that I am reading is part of a trilogy.

"where" for places where an activity takes place

• A post office is a place where you can buy stamps.
• This is the restaurant where we had our first date.
• Perth is the city where I grew up.

"whose" for possessives

• Jason is the boy whose parents own the newsagent.
• That's the girl whose mother is a ballet dancer.

"when" for times

• My grandmother lived at a time when women were expected to become either teachers or nurses.

Note : We don't use "what" in relative clauses!

eg. These are the jeans that I want to buy. NOT These are the jeans what I want to buy.

In defining relative clauses, we can leave out "who", "that" or "which" when these words aren't the subject of the defining relative clause.

• He's the man (that) I met yesterday.
• Mary found the watch (that) I lost last week.

We never leave out "whose'" in defining relative clauses. We can leave out "where" if we add a preposition at the end of the relative clause.

• That's the city where I was born. = That's the city I was born in.

We can leave out "when" if the time reference is clear.

• Next Tuesday is the day (when) my sister arrives.

Note : We don't use commas with defining relative clauses.