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.
注意 ：
(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.
(3)关系代词在句中作主语时，从句的谓语动词的人称和数必须和先行词保持一致。
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.
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?
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.
→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。
（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.
→The man standing under the tree is our English teacher.
站在树下面的那个人是我们的英语老师。
I saw the house that was burning at that time.→I saw the burning house at that time.
当时我看到那房子在燃烧。
（3）定语从句简化为过去分词短语作后置定语。
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。
（4）定语从句简化为不定式作后置定语。
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。
（5）定语从句简化为what 从句。
I couldn't remember the words that he said.→I couldn't remember what he said.
我记不得他说的话。
【典型例题解析】
例 1 The second book ______I want to read is Business @ the Speed of Thought.
A. which B. what C. that D. as
解析 先行词 book被序数词修饰时要用that引导定语从句，故选C。
例 2 I'll never forget the days _____I stayed with you.
A. when B. in which C. that D. for which
解析 本题指时间，故选 A。
例 3 The book______ is sold out at the moment.
A. you need B. what you need
C. which you need it D. that you need it
解析 B、C、D中的what和it与先行The book相抵触 ， 故选 A。
例 4 Is this the place ______Lincoln once lived.
A. that B. which C. where D. when
解析 本题指地点，故选 C。
例 5 I'm one of the boys _________ never late for school.
A. that is B. who are C. who am D. who is
解析 本题中 who用作主语，谓语动词与先行词the boys保持一致，故选B。
【选讲例题】
例 6 Her sister,______ you met at my home, was a teacher of English.
A. whom B. that C. which D. who is
解析 非限制性定语从句中 ， 关系代词用 which,不用that,但指人时用who或whom.故选A。
例 7 These book are for students _____ mother language is not English.
A. of whom B. that C. which D. whose
解析 whose引导的定语从句中,whose用作定语，故选D。