We use relative clauses to give additional information about something without starting another sentence. By combining sentences with a relative clause, your text becomes more fluent and you can avoid repeating certain words.
1. How to Form Relative Clauses:
A woman is sitting next to Peter.
You want to know who she is and ask a friend whether he knows her.
You could say: A woman is sitting next to Peter. Do you know that woman?
That sounds rather complicated, doesn't it? It would be easier with a relative clause: you put both pieces of information into one sentence. Start with the most important thing – you want to know who the woman is.
Do you know the woman? As your friend cannot know which person you are talking about, you need to put in the additional information – the woman is sitting next to Peter.
Use "the woman" only in the first part of the sentence, in the second part replace it with the relative pronoun (for people, use the relative pronoun "who").
So the final sentence is: Do you know the woman who is sitting next to Peter?
||Subject or object pronoun for PEOPLE. ||I've got a friend who
lives in Bath.|
||Subject or object pronoun for ANIMALS and THINGS. ||Can you see the pen which
is under your chair?|
||Referring to a whole sentence. ||She couldn't speak any English which
||Possession for PEOPLE, ANIMALS and THINGS. ||Do you know the boy whose
father is a policeman?|
||Object pronoun for PEOPLE, especially in non-defining relative clauses.
In defining relative clauses we colloquially prefer WHO. ||I was invited by the head manager whom
I met at the conference in Liverpool.|
||Subject or object pronoun for PEOPLE, ANIMALS and THINGS in defining relative clauses. (WHO or WHICH are also possible) ||I don't really like the car that
he has just bought.|
2. Subject Pronoun or Object Pronoun?
Subject and object pronouns cannot be distinguished by their forms - who, which and that are used for subject and object pronouns. You can, however, distinguish them as follows:
If the relative pronoun is followed by a verb, the relative pronoun is a subject pronoun. Subject pronouns must always be used.
Example: the ball which is under your car.
If the relative pronoun is not followed by a verb (but by a noun or pronoun), the relative pronoun is an object pronoun. Object pronouns can be omited in defining relative clauses, which are then called Contact Clauses.
Example: the ball (which) the children Smith put under your car.
3. Relative Adverbs:
A relative adverb can be used instead of a relative pronoun plus preposition. This often makes the sentence easier to understand.
Example: This is the shop in which I bought my bike.
Example: This is the shop where I bought my bike.
||IN / ON WHICH ||It refers to a TIME EXPRESSION. ||The day when
I saw her.|
||IN / AT WHICH ||It refers to a PLACE. ||The place where
I met her.|
||FOR WHICH ||It refers to a REASON. ||The reason why
I met her.|