Some relative pronouns introduce relative clauses (‘relativsetninger’) which make it clear what exactly the head noun refers to; in the example below which type of teachers. In such cases the clause is necessary. We call it restrictive.

If the relative clause adds extra information about the head; here: all teachers, it is unnecessary. We call it non-restrictive. Compare:

Restrictive and non-restrictive
Teachers who have a sense of humour will be chosen.	(Only those with a sense of humour...)
Teachers, who have a sense of humour, will be chosen.	(All teachers have a sense of humour and they will be chosen.)

Non-restrictive, but not restrictive clauses, should be set off by commas.

Use of relative pronouns

Who and that are used to refer to people, but that should only be used in restrictive clauses. We can leave out the relative pronoun – use the zero form - if it is not the subject in the clause.

I met a nice Italian who /that told me everything. (restrictive relative clause)
My uncle, who is Italian, told me everything. (non-restrictive clause)
He is the actor [Ø] I told you about.

Whom must be used after prepositions, but is otherwise considered formal. Like who it refers to people.

We’ll send it to whom it may concern.
There were at least fifty people in the group, of whom more than half were immigrants.

Which, that and zero [Ø] (in non-subject position) are used to refer to inanimate (non-living) objects.

Which, preceded by a comma, must be used if the relative clause refers to the content of the clause before.

I had run into a situation which/that/Ø I had never experienced before.
Courses which are offered on a regular basis attract plenty of students.
He decided to quit his job, which surprised everybody.

What fuses ‘that which’ and is strictly speaking not a normal relative pronoun.

Nobody seems to understand what hit them.

When, where and why work as relative markers of nouns that refer to time, place and reason.

At times when everybody had given up, Frank persisted.
I always prefer places where I can find my way around.
She couldn’t see why the others were so upset.

Note: Whose is not a pronoun but a relative determiner because it can only occur in front of nouns. As a determiner it can be used with personal and non-personal heads.

John is a friend whose help I can always depend on.
You need a car whose engine is strong enough for the uphill climbs

Replacing Norwegian "som" (Relative Pronouns)