定语从句（Attributive Clauses）在句中做定语，修饰一个名词或代词，有时也可以修饰部分或整个句子。
　　被修饰的名词，词组或代词即先行词。定语从句通常出现在先行词之后，由关系词（关系代词或关系副词）引出。
　　关系代词有：who, whom, whose, that, which, as。
　　关系副词有：when, where, why, how。
　　关系代词和关系副词放在先行词和定语从句之间，起连接作用，同时又可做定语从句的一个成分。当关系代词做宾语时可以省略。
　　定语从句中的谓语动词必须在人称上和数量上和先行词保持一致。
　　定语从句分为限制性定语从句和非限制性定语从句。

1 、关系代词引导的定语从句
1) who, whom, that 这些词代替的先行词是人的名词或代词，在从句中所起作用如下：
Is he the man who/that wants to see you?（who/that在从句中作主语）
He is the man whom/ that I saw yesterday.（whom/that在从句中作宾语）
2) whose 用来指人或物，（只用作定语，若指物，它还可以同of which互换），例如：　
Please pass me the book whose (of which) cover is green.　
3) which, that 它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等，例如：
A prosperity¹ which / that had never been seen before appears in the countryside.（which / that在句中作宾语）
The package (which / that) you are carrying is about to come unwrapped. (which / that在句中作宾语)

关系代词that和which 都可以指物，that 和Who 都可以指人，其用法区别：

不用that的情况：

a) 在引导非限定性定语从句时
(错)The tree, that is four hundred years old, is very famous here.
b) 介词后不能用
We depend on the land from which we get our food.
c) 多用who 的情况
①关系代词在从句中做主语
A friend who helps you in time of need is a real friend.
②先行词为those, people 时
Those who were either fools or unfit² for their offices could not see the cloth.
③先行词为all, anyone, ones, one 指人时
One who doesn't work hard will never succeed in his work.
④在There be句型中
There is a stranger who wants to see you.
⑤在被分隔的定语从句中
A new teacher will come tomorrow who will teach you German.
⑥在有两个定语从句的句子中，其一用who，其二用that,但若先行词后接两个以上的并列定语从句时，后一个必须重复前一个关系代词。
The student who was praised at the meeting is the monitor that is very modest and studies very hard.
There is a teacher who is always ready to help others and who enjoys what he does.

2) 只能用that作为定语从句的关系代词的情况
a)在不定代词，如：anything, nothing, everything, all, much, few, any, little等作先行词时，只用that，不用which。
All that is needed is a supply of oil.
Finally, the thief handed everything that he had stolen to the police.　
b)先行词有the only, the very, the just修饰时，只用that。　
 He is the very man that helped the girl out of the water.
c)先行词为序数词(the last)、数词、形容词最高级时，只用that。
The first English book that I read was "The Prince and the Pauper³" by Mark Twin.
d)先行词既有人，又有物时。
He talked about the teachers and schools that he visited.
e)当主句是以who 或which 开始的特殊疑问句时，用that 以避免重复。
Who is the person that is standing⁴ at the gate.
f)关系代词在从句中做表语
He is not the man that he used to be.

2 、关系副词引导的定语从句
关系副词可代替的先行词是时间、地点、方式或理由的名词，在从句中作状语。
关系副词when, where, why, how的含义相当于"介词+ which"结构，因此常常和"介词+ which"结构交替使用，例如：
There are occasions⁵ when (on which) one must yield⁶.　
Beijing is the place where (in which) I was born.　　
Is this the reason why (for which) he refused our offer?　
I'm surprised the way how (by which) he works⁷ out the problem.

注意：

①在非限制性定语从句中，"介词+ which"结构不能代替关系副词。
如：They set up a state for their own , where they would be free to keep Negroes⁸ as slaves⁹.
②含有介词短语的动词一般不能拆开，介词仍放在动词后面。
Is this the book which (that) she was looking for?

3、名词/数词/代词 /形容词最高级 + 介词 + 关系代词引导定语从句
She has written a book , the name of which I have forgotten.
There are fifty-five students in our class , all of whom are working hard.
There are five continents in the world , the largest of which is Asia.

4、as, which 引导非限定性定语从句的差别
由as, which 引导的非限定性定语从句，as和which可代整个主句，相当于and this或and that。As一般放在句首，which在句中。
As we know, smoking¹⁰ is harmful to one's health.
The sun heats the earth, which is very important to us.　　
as可引导非限制性从句，常带有“正如”的意思。
As is know, smoking is harmful to one's health.

用法区别：

(1) as 引导的定语从句可置于句首，而which不可。
As we all know, he never smokes.
(2) as 代表前面的整个主句并在从句中作主语时，从句中的谓语必须是系动词；若为行为动词，则从句中的关系代词只能用which。
(3)非限定性定语从句中出现expect, think, suppose 等表示猜测、想象、预料等时。
She succeeded in her doing the research work , as we expected.
(4)As 的用法 the same… as; such…as 中的as 是一种固定结构, 和……一样……。
I should like to use the same tool as is used here.
We should have such a dictionary as he is using.

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