1.who指人，在从句中做主语
(1)TheboyswhoareplayingfootballarefromClassOne.
(2)YesterdayIhelpedanoldmanwholosthisway.
2.whom指人，在定语从句中充当宾语，常可省略。
(1)Mr.Liuistheperson(whom)youtalkedabout.
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)Themanwho/whomyoumetjustnowismyfriend.
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)Footballisagamewhichislikedbymostboys.(which在句子中做主语)
(2)Thisisthepen(which)heboughtyesterday.(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)Thepeoplethat/whocometovisitthecityareallhere.(在句子中做主语)
(6)Whereisthemanthat/whomIsawthismorning?(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)Hehasafriendwhosefatherisadoctor.
(2)Ilivedinahousewhoseroofhasfallenin.
whose指物时，常用以下结构来代替
(3)Theclassroomwhosedoorisbrokenwillsoonberepaired.
=Theclassroomthedoorofwhichisbrokenwillsoonberepaired.
(5)Doyoulikethebookwhosecolorisyellow?
=Doyoulikethebookthecolorofwhichisyellow?