Can you give me back the money which I lent you last month?
(Use "which" for things or objects.)

This is the restaurant where we used to eat when we lived in Boston.
(Use "where" for places.)

Mark has sent me an e-mail which I haven't replied to yet.
(Use "which" for things.)

Who's the person who is sitting next to Nancy?
(Use "who" for people, as the subject of the relative clause.)

They complained about the wrong goods which were sent to them.
(Use "which" for things.)

This is Susan whose husband works in the sales department.
(Use "whose" to show possession.)

Candy is wearing a new dress which she bought in the summer sales.
(Use "which" for things.)

Monday is the day when bills have to be paid.
(Use "when" for time or specific days.)

The secretary showed me the filing cabinet where important documents are filed.
(Use "where" for places.)

Do you like the boy whom Mary is talking to?
(Use "whom" when the relative pronoun is the object of the verb in the relative clause. It can be replaced by "who" in informal contexts.)