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：
The room in which there is a machine is a work shop。
The river which is in front of my house is very clean。
This is the pen which you want。
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。
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主句先行词的补充说明 ， 没有这种从句 ， 不影响主句意思的完整 ， 一般用逗号把主句和从句分开 ， 关系代词用 which,不用that;指人时可用who,如 ： I have two brothers, who are both students。
8、如何简化定语从句
（1）.定语从句简化为形容词或形容词短语作后置定语。如:
My grandfather lives in a village that is far away from here。
（2） 定语从句简化为现在分词或现在分词短语作前置或后置定语。
The man who is standing under the tree is our English teacher。
扩展资料
注意在下面几种情况下必须用 that引导定语从句。
(1)先行词是不定代词all, few, little, much, something, nothing, anything等 ， 如 ：
All that we have to do is to practise English。
(2)先行词被序数词或形容词最高级所修饰，如
The first letter that I got from him will be kept。
(3)先行词被all, any, every, each, few, little, no, some等修饰 ， 如
I've eaten up all the food that you gave me。
(4)先行词被the only, the very, the same, the last修饰时如
He is the only person that I want to talk with。
(5)先行词既有人又有物时，如:
They talked about persons and things that they met。
(6)当句中已有who时 ， 为避免重复 ， 如 ： Who is the man that is giving us the
class?
百度百科-定语从句