一、定语从句
在复合句中， 修饰名词或代词的从句叫定语从句，被修饰的名词或代词叫先行词，引导定语从句的有关系代词 who, whom, whose, which, that等和关系副词where, when, why等，关系代词和关系副词在定语从句中担任句子成份。
1.由who引导的定语从句中，who用作主语， 如：This is the boy who often helps me.
2.由whom引导的定语从句中，whom用作宾语，如：The man whom you are waiting for has gone home.
3.由whose引导的定语从句中，whose用作定语 ，如 ：Do you know the girl whose skirt is white?
4.由which引导的定语从句中，which用作主语或谓语动词的宾语或介词的宾语，如：
5.由that引导的定语从句中，that可以指人或物，在从句中作主语或谓语动词的宾语，但不能放在介词后面作介词宾语，如：
The book that I bought yesterday was written by Lu Xun.
6.由when, where, why引导的定语从句，如:
I don't know the reason why he was late.
This is the place where we have lived for 5 years.
I'll never forget the day when I met Mr Li for the first time.
注意：先行词是表示地点时，如果从句的谓语动词是及物的，就用 that(which)，如果从句的谓语动词是不及物的，就用where引导。This is the house Which /that he has lived in for 15 years.(Where he has lived for 15 year.)
7. 限制性定语从句和非限制性定语从句
(1)限制性定语从句是句中不可缺少的组成部分，主句和从句之间不用逗号分开。引导非限制性定语从句的关系代词有who, whom, whose, which, of which等，这些关系代词都不能省略。
(2)非限制性定语从句是对。
