Relative pronouns

who-->as the subject or object of a relative clause
The students who cheated in the exam were punished.
(who=the students=subject of the relative clause)

I have just met a friend who I have not seen for a long time.
(who=a friend=object of the verb have seen)

whom-->only as the object of a relative clause
The beggar whom the children often laugh at is eating a piece of rotten bread.
(whom=the beggar=object of the preposition at)

The little girl whom the dog bit is from a rich family.

whose-->shows that something belongs to someone
Do you know whose fault it is?
I give some money to the man whose shirt is torn.

which-->refers to things and animals
Children like to keep pets which are cute.
The fast food shop which I went to yesterday has many branches.