1.who指人，在从句中做主语
(1)The boys who are playing football are from Class One.
(2)Yesterday I helped an old man who lost his way.
2. whom指人，在定语从句中充当宾语，常可省略。
(1) Mr. Liu is the person (whom) you talked about.
注意：关系代词whom在口语和非正式语体中常用who代替，可省略。
(3) The man who/whom you met just now is my friend.
3. which指物，在定语从句中做主语或者宾语，做宾语时可省略
(1) Football is a game which is liked by most boys. ( which 在句子中做主语)
(2) This is the pen (which) he bought yesterday. ( which 在句子中做宾语)
4. that指人时，相当于who或者whom;指物时，相当于which。
在宾语从句中做主语或者宾语，做宾语时可省略。
(5) The people that/who come to visit the city are all here. (在句子中做主语)
(6) Where is the man that/whom I saw this morning? (在句子中做宾语)
5. whose通常指人，也可指物，在定语从句中做定语
(1) He has a friend whose father is a doctor.
(2) I lived in a house whose roof has fallen in.
whose指物时，常用以下结构来代替
(3)The classroom whose door is broken will soon be repaired.
=The classroom the door of which is broken will soon be repaired.
(5)Do you like the book whose color is yellow?
=Do you like the book the color of which is yellow?