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1) Definition

A clause is a part of a sentence. A relative clause tells us which person or thing the speaker means.

Ex : People who live in London are happy.

"who live in London" tells us what kind of people are happy.

2) Who-Whom-Which

A) Who

We use "who" in a relative clause when we are talking about people (not things). We use "who" instead of he / she / they.

Ex : The woman lives in this house. She is a doctor
The woman who lives in this house is a doctor.

You must use "who" when it is the subject of the relative clause. But when "who" introduces the object, you can leave it.
So you can say :
The woman who I wanted to see was away
Or The woman I wanted to see was away.

B) Whom

"Whom" is more correct than "who" when it is the object of the verb.

Ex : the woman whom I wanted to see was away.

C) Which
When we are talking about things, we use "which" in a relative clause. We use "which" instead of it/they.

Ex : I hate the cheese. It was in the fridge.
I hate the cheese which was in the fridge.

Like "who", you must use "which" when it is the subject of the relative clause and can leave it when it is the object.

Ex : I hate the cheese which was in the fridge.
I have just found the keys (which) I lost yesterday.

3) Whose-Where

A) Whose

We use "whose" in a relative clause instead of his/her/their to express possession.

Ex : Yesterday I met a man. His wife is an English teacher.
Yesterday I met a man whose wife is a English teacher.

Be careful with "who" and "whose"

Ex : I met a man who knows you : he knows you
I met a man whose sister knows you : his sister knows you.

B) Where

You can use "where" in a relative clause to talk about a place.

Ex : The town where I live is in the south of France.

4) What-which

In relative clauses, "what" announces something whereas "which" resumes something.

Ex : What surprises me was that Andy failed his exams.
Andy failed his exams, which surprised me.

5) Translate :

1) Je connais une femme qui possède un restaurant.
I know a woman who owns a restaurant

2) Le film que j'ai vu hier était ennuyeux
The film (which) I saw yesterday was boring

3)Ce qui est dommage est que Jeanne n'ait pas pu aller à la fête.
What is a pity is that Jeanne couldn't go to the party

4) Une veuve est une femme dont le mari est mort.
A widow is a woman whose husband is dead

5) Les enfants que j'ai vus jouaient au football.
The children (who(m)) I saw played football.

6) Il y a beaucoup de bruit la nuit, ce qui m'empêche de dormir.
There is much noise at night, which prevents me from sleeping.