Relative Pronouns問題

that常在口語時用，可以代替who,不過有時候會看情況用。

當定語從句是形容主詞時（即subject):
The woman who lives next door is a doctor.
= The woman that lives next door is a doctor.
(The woman is a doctor. She lives next door.)

當定語從句是形容賓語時（即object):
The woman that/who I wanted to see was away.
=The woman I wanted to see was away.

當從句是非定語從句時（即額外給予信息，以'，'分開）：
My brother Jim, who lives in London, is a doctor.
(這種情況一定不能用that)

另外，that可以代替which, why:
Are these the keys (that/which) you were looking for?
The reason (that/why) I am phoning you is to invite you to a party.

That can be the object of a preposition.

I found the following sentence in Oxford Advanced Learner's English-Chinese Dictionary.
It is a different kind of car from that (which) I am used to.