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 on the bus.
(2) Mr.Ling is just the boy whom I want to see.
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.
(2) This is the pen (which) he bought yesterday.
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 number of the people that/who come to visit the city each year rises one million.
(6) Where is the man that/whom I saw this morning?
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 once 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.
(4) The classroom the door of which is broken will soon be repaired.
(5) Do you like the book whose cover is yellow?
(6) Do you like the book the color of which is yellow?