WHAT ARE RELATIVE CLAUSES?
As the name suggests, these clauses give essential information to define or identify the person or thing we are talking about. Obviously, this is only necessary if there is more than one person or thing involved.
Elephants who marry mice are very unusual.
In this sentence we understand that there are many elephants, but it is clear that we are only talking the ones who marry mice.
Commas are not used in defining relative clauses.
The following relative pronouns are used in defining relative clauses:
1. The relative pronoun stands in place of a noun.
This noun usually appears earlier in the sentence:
The woman who/that spoke at the meeting was very knowledgeable.
2. Who, whom and which can be replaced by that. This is very common in spoken English.
3. The relative pronoun can be omitted (ø) when it is the object of the clause:
The mouse that the elephant loved was very beautiful.
OR The mouse the elephant loved was very beautiful.
Both of these sentences are correct, though the second one is more common in spoken English.
The mouse that/ø the elephant loved was very beautiful.
(You can usually decide whether a relative pronoun is an object because it is normally followed by another subject + verb.)
4. Whose is used for things as well as for people.
The man whose car was stolen.
A tree whose leaves have fallen.
5. Whom is very formal and is only used in written English. You can use who/that, or omit the pronoun completely :
The doctor whom/who/that/ø I was hoping to see wasn't on duty.
6. That normally follows words like something, anything, everything, nothing, all, and superlatives.
There's something that you should know.
It was the best film that I've ever seen.
A clown is someone who makes you laugh.
An elephant is an animal that lives in hot countries.
The plums that were in the fridge were delicious. I have eaten them.
Where are the plums (that) I put in the fridge?
Has anyone seen the book I was reading?
Nothing that anyone does can replace my lost bag.
Let's go to a country where the sun always shines.
They live in the house whose roof is full of holes.