定语从句
I. 定语从句起了形容词的作用，在句中修饰一个名词或代词。被修饰的词叫做先行词，引导定语从句的词叫关系词，他的作用一是放在先行词与定语从句中间起了连接作用，二是在从句中担当一个成分，并与先行词保持数的一致。
关系词
先行词
从句成分
例句
备注
关系代词
who
人
主语
Do you know the man who is talking with your mother?
whom, which和that在从句中做宾语时，常可以省略，但介词提前时后面关系代词不能省略，也不可以用that
whom
人
宾语
Mr. Smith is the person with whom I am working
The boy (whom) she loved died in the war..
whose
人，物
定语
I like those books whose topics are about history.
The boy whose father works abroad is my deskmate.
that
人，物
主语，宾语
A plane is a machine that can fly.
She is the pop star (that) I want to see very much.
which
物
主语，宾语
The book (which) I gave you was worth $10.
The picture which was about the accident was terrible.
as
人，物
主语，宾语
He is such a person as is respected by all of us.
This is the same pen as I lost yesterday.
as做宾语一般不省略
关系副词
when
时间
时间状语
I will never forget the day when we met there.
可用on which
where
地点
地点状语
This is the house where I was born.
可用in which
why
原因
原因状语
I can’t imagine the reason why he turned down my offer.
可用for which
II. that与which, who, whom的用法区别：
情况
用法说明
例句
只用that的情况
1． 先行词为all, everything, anything, nothing, little, much,等不定代词时。
2． 先行词被all, any, every, each, much, little, no, some, few等修饰时
3． 先行词有形容词最高级和序数词修饰时
4． 先行词既指人又指物时
5． 先行词被the only, the very修饰时
6． 句中已经有who或which时，为了避免重复时
1.He told me everything that he knows.
2.All the books that you offered has been given out.
3.This is the best film that I have ever read.
4.We talked about the persons and things that we remembered.
5.He is the only man that I want to see.
6.Who is the man that is making a speech?
只用which, who, whom的情况
1． 在非限制性定语从句中，只能用which指代物，用who/whom指人
2． 在由“介词+关系代词”引导的定语从句中，只能用which指物，whom指人。
3． 先行词本身是that时，关系词用which, 先行词为those, one, he时多用who。
He has a son, who has gone abroad for further study.
I like the person to whom the teacher is talking.
Those who respect others are usually respected by others.
III. as与which的区别：
定语从句
区别
例句
限制性定语从句中
名词前有such和the same修饰时，关系代词用as,不能用which
He is not such a fool as he looks.
Don’t read such books as you can’t understand.
非限制性定语从句中
as和which都可以指代前面整个主句。如果有“正如，象”的含义，并可以放在主句前，也可以放在后面，那么用as；而which引导的从句只能放主句后，并无“正如”的意思。
They won the game, as we had expected.
They won the game, which we hadn’t expected.
As is well known, he is a famous film star in the 1980s.
IV. 限制性定语从句与非限制性定语从句的区别：
类别
意义及特征
例句
限制性定语从句
对先行词起修饰限制作用，如果去掉，主句意思就不完整明确，这种从句与主句的关系十分密切，写时不用逗号分开。
The accident happened at the time when I left.
非限制性定语从句
对先行词作附加的说明，与主句的关系不十分密切，较松散。从句和主句之间用逗号分开，相当于一个插入语，不能用that引导，关系代词做宾语时也不能省略。
His mother, whom he loved deeply, died ten years ago
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