Animals and things ( subject of verb / object of verb )
Example: That´s the dog which ate my lunch.
To replace who, which, whom
Exemple: I prefer dogs that don´t bark a lot.
For people and non-people ( possessive case )
Example: I have a friend whoose mother is portuguesse.
For people ( subject of verb )
Example: A doctor is a person who works in a hospital.
For people ( object of verb )
Example: That´s the women whom I call a few minutes ago.
Exemple : When is the meeting?
Exemple : Where are my keys?