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?
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.
(1)whom, which用作介词宾语时,介词可放在 whom、which之前,也可放在从句原来的位置上;但在含有介词的动词固定词组中，介词只能放在原来的.位置上。如： He is the very person whom we must take good care of.
(2)引导非限制性定语从句时，必须用关系代词which，不用that,如：I have lost
my bag, which I like very much.
The book that I bought yesterday was written by Lu Xun.
(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.
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.
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
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.)
(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.
My grandfather lives in a village that is far away from here.
→My grandfather lives in a village far away from here.我祖父住在离这儿很远的一个村子。
This is a book that is worth reading.
→This is a book worth reading. 这是一本值得看的书。
The man who is standing under the tree is our English teacher.
→The man standing under the tree is our English teacher.
I saw the house that was burning at that time.→I saw the burning house at that time.
I like to see the films which are directed by Zhang Yimou.→I like to see the films directed by Zhang Yimou. 我喜欢看张艺谋导演的电影。
She is the girl who was praised at the school meeting.→She is the girl praised at the school meeting. 她就是在校会上受表彰的那个女孩。
He is always the first person that comes to school.→He is always the first person to come to school.他总是第一个到校。
The report which will be given tomorrow is important to us.→The report to be given tomorrow is important to us. 明天要作的报告对我们很重要。
I couldn't remember the words that he said.→I couldn't remember what he said.