The jungle (that) the tribe lived in was full of strange and unusual animals. The jungle in which the tribe lived was full of strange and unusual animals.

Informal: The hotel room (which/that) we are staying in is really big Formal: The hotel room in which we are staying is really big

You can say what/whatever you like, there us no substitute for hard work

Informal: The house (which/that) Mozart was born in is now a museum Formal: The house in which Mozart was born is now a museum

You should do whatever makes you happy

Informal: Do you know the girl (who/that) Tom is talking to? Formal: Do you know the girl to whom Tom is talking to?

You can do it however you like