Relative pronouns (in Dutch: betrekkelijke voornaamwoorden) are used in subordinate conjuctions, to refer to something or someone in de coordinate conjunction:
Who refers to people:
- This is the girl who helped me.
- Those people, who looked at us angrily, didn't agree with our statement.
Which refers to animals and things:
- The crow, which was sitting on its nest, was feeding his babies.
- This is a picture of the house, which my father built all by himself.
That can be used to replace 'who' and 'which'. This can only be used in subordinate sentences that cannot be removed, because it would interfere with the meaning/sense of the sentence. Should you use 'that', there's no comma before this word:
- The man who/that helped us was very nice.
- This is a picture of the house which/that my father built.
Whose can refer to people, animals and things and it indicates possession:
- The man whose glasses are broken is in the store now.
- The lion whose cubs are ill, is in quarantine.
- The plant whose leafs are green isn't going to survive much longer.
Whom refers to people and is used after prepositions such as with, to, about, for, etc. It's quite formal and you'd therefore more often hear 'who' instead of 'whom'.
- That's the man to whom they gave my function.
- That's the person to whom I sent that email.