一. 定义：在复合句中，修饰名词或代词的从句；被修饰的名词或代词叫做先行词，引导定语从句的词叫关系词，关系词分为关系代词和关系副词；定义从句相当于形容词的用法。
二. 关系代词:
1. 作用：引导定语从句，指代先行词并且在从句中作成分，关系代词只能作主语，宾语，定语，表语；
2.分类：根据先行词指人还是指物可以分为两类。
(1)指人的关系代词有who, whom, whose, that；
①who在定语从句中作主语，不可省略；
②whom在定语从句中作宾语，可以省略，但若whom直接放在介词之后则不可以省略；
③whose在定语从句中作定语，后面必须有个名词，先行词和这个名词之间存在所属关系。
④that可作主语，宾语，表语，可以和who, whom互换；不可作定语，不能与whose互换。
(2)指物的关系代词有which, whose, that;
①which在定语从句中既可作主语，也可作宾语；作主语时，不可省略；作宾语时，可以省略；
②whose作定语，后面一定有格名词，先行词和这个名词构成所属关系.
③that可作主语，宾语，表语，可以和which互换；不可作定语，不能与whose互换。
e.g. The man who lives next to us sells vegetables.
You must do everything that I do.
3. who, whom引导的定语从句:
(1) who，whom用来代替人，who是主格，在定语从句中作主语，不可省略；whom为宾格，在定语从句中作宾语，可省略；who和whom都可以与that互换；
e.g. The person who is watching TV is my father.
The boy (whom) you saw yesterday is here now.
(2)注意：口语中可用who代替whom，但是如果关系代词作介词的宾语时，只能用whom，如果介词在词尾，两者都可以用。
e.g. This is the girl (who/whom) he worked with.
This is the girl with whom he worked.
4. whose 引导的定语从句:
whose既可以指人，也可指物，指物时可以和of which互换；whose是所有格，在定语从句中作定语，不可省略，不可与that互换。
e.g. This is the scientist whose name is known all over the world.
The room whose window faces the south is mine.
People whose dogs bite other people should keep them tied up.
5. which引导的定语从句:
(1)which指物，在定语从句中作主语和宾语，作主语时不可省略，作宾语时可省略，which可与that互换;
e.g. The car which was stolen has been found.
The few points (which) the president stressed in his report are very important.
(2)注意：which除了指物以外，还可以指婴儿，动物等。
e.g. The baby (which) the nurse brought in was born yesterday.
6. that引导的定语从句:
that既可以指人也可以指物，在定语从句中作主语，宾语或表语，不可作定语；作宾语或表语时可以省略，作主语时不可省略。
e.g. A plane is a machine that can fly.
The noodles (that) I cooked were delicious.
Who is the man that is reading a book over there.
The girl (that) we saw yesterday is Mary’s sister.
7. 通常只用关系代词that，不用who, whom, which的 几种情:
(1)当先行词为不定代词（all, everything, little, none, much,few等）时，定语从句用that引导；
e.g. Is there anything that I can do for you?
Much that I used in the book is very useful.
(2)当先行词被序数词或者形容词，副词的最高级修饰时，定语从句常用that引导；
e.g. The first thing that I’m going to do tonight is to write to my parents.
This is the most interesting film that has been produced in recent years.
(3)当先行词被no, every, some, any, only, very,much等限定词修饰时，定语从句常用that引导；
e.g. This is the very book that I have been looking for.
There is no person that doesn’t make mistakes.
That’s the only thing that we can do now.
(4)当并列的两个先行词既有人也有物时，定语从句用that引导；
e.g. They are talking about the teachers and schools that they have visited.
Look at the man and the donkey that are walking along the street.
(5)当关系代词在定语从句中作表语时，用that引导；
e.g. He doesn’t seem to be the man that he was.
(6)当定语从句为there be句型时，从句主语的关系代词只能用that,但通常省略；
e.g. I know the difference (that)there is between you.
The number of mistake (that) there are in his homework is simply surprising.
(7)注意：当先行词是人称代词和指示代词时，常用who引导。
e.g. He who contributes to the people deserves respect.
Those who don’t want to go can stay at the classroom.
8.限制性定语从句中不用that的情况：关系代词在定语从句中作定语时，只用whose,不用that。
e.g.Tom is the boy whose mother helped me yesterday.
I like the book whose pictures are beautiful.
