一、定语从句的定义：
在复合句中,修饰某一名词或代词的句子叫定语从句。被修饰的词叫先行词。定语从句不同于单词作定语的情况，它需要放在被修饰的词（即先行词）之后。定语从句由关系词（关系代词、关系副词）引导，关系代词、关系副词位于定语从句句首。
下面先随着三思小编来了解关涉定语从句的几个小知识：
1、定语：定语用来限定、修饰名词或代词，是对名词或代词起修饰、限定作用的词、短语（动词不定式短语、动名词表示的）常用“…的”表示。主要由形容词担任，此外，名词、代词、数词、分词、副词、不定式以及介词短语也可以来担任，也可以由一个句子来担任。单词作定语时通常放在它所修饰的词之前，作前置定语。短语、从句作定语时则放在所修饰的词之后，作后置定语。
2、先行词：指被定语从句修饰的名词、代词。一般先行词出现在定语从句的前面。
3、关系词：引导定语从句的词称为关系词。关系词包括关系代词、关系副词。关系词常有2个作用： ①连接作用，引导定语从句。 ②代替先行词，在定语从句中担当一个成分。
4、关系代词：关系代词有主语、宾语、定语之分。一般who做主语或其宾格形式的whom做宾语(whom作宾语时可省略），whose作为定语（whose不可省略）。关系代词在从句中作主语，宾语，定语等，关系副词在从句中作地点状语（where），时间状语（when），原因状语（why）。
关系代词所代替的先行词是人或物的名词或代词，并在从句中充当主语、宾语、定语等成分。关系代词在定语从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。
1、who, whom, that
限定性定语从句中的关系代词
作主语
作宾语
作定语
指人
who/that
whom/that(可省略)
whose
指物
which/that
which/that(可省略)
whose
指人和物
that
that
whose
非限定性定语从句中的关系代词
作主语
作宾语
作定语
指人
who
whom
whose/of whom
指物
which
which
whose/of which
4、关系副词：关系副词在句中作状语。关系副词=介词+关系代词。
1）why=for which主要用于修饰表原因的名词（主要是the reason）同时它在定语从句中用作原因状语
We don’t know the reason why he didn’t show up. 我们不知道他为什么没有来。
·与关系副词when和where不同，why可以换成that或省略。如：
That’s one of the reasons (why, that) I asked you to come. 这就是我请你来的原因之一。
·与关系副词when和where可以引导非限制性定语从句不一样，why只能引导限制性定语从句，不能引导非限制性定语从句。如：
他失去工作的主要原因是他喝酒。
误：The main reason, why he lost his job, was that he drank.
正：The main reason why he lost his job was that he drank. (同位语从句)
2）where=in/ at/ on which（介词同先行词搭配）主要用于修饰表时间的名词，同时它在定语从句中用作时间状语。如：
There comes a time when you have to make a choice. 你必须作出抉择的时候到了。
·不要一见到先行词为时间名词，就以为一定要用关系副词when来引导定语从句，同时还要看它在定语从句中充当什么成分——如果在定语从句中用作时间状语，就用when；如果在定语从句中不是用作时间状语，而是用作主语或宾语，那就不能用when，而要用that, which等。如：
Don’t forget the time (that, which) I’ve told you. 不要忘记我告诉你的时间。
（讲解：关系代词that / which在定语从句中用作told的宾语，正因为用作宾语，故可省略。）
3）when=during/ on/ in/… 主要用于修饰地点名词，同时它在定语从句中用作地点状语。如：
This is the village where he was born. 这就是他出生的村子。
That’s the hotel where we were staying last summer. 这就是我们去年夏天住的旅馆。
·与前面when的情况一样，注意不要一见到先行词为地点名词，就以为一定要用关系副词where来引导定语从句，同时还要看它在定语从句中充当什么成分。如果在定语从句中用作地点状语，就用where；如果在定语从句中不是用作地点状语，而是用作主语或宾语，那就不能用where，而要用that, which等。 如：He works in a factory that /which makes TV sets. 他在一家电视机厂工作。
二、定语从句的类别：
1、关系代词引导的定语从句
关系代词所代替的先行词是人或物的名词或代词，并在句中充当主语、宾语、定语等成分。关系代词在定语 从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。
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 you want to see?
他就是你想见的人吗？（who/that在从句中作主语）
He is the man whom/ that I saw yesterday.
他就是我昨天见的那个人。（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：
They rushed over to help the man whose car had broken down.
那人车坏了，大家都跑过去帮忙。
Please pass me the book whose (of which) cover is green. 请递给我那本绿皮的书。
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 prosperity which / that had never been seen before appears in the countryside.
农村出现了前所未 有的繁荣。（which / that在句中作主语）
The package which / that you are carrying is about to come unwrapped.
你拿的包快散了。(which / that 在句中作宾语）
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、地点或理由的名词，在从句中作状语。
1）when, where, why，其含义相当于"介词+ which"结构，因此常常和"介词+ which"结构交替使用，例 如：
There are occasions when (on which) one must yield. 任何人都有不得不屈服的时候。
Beijing is the place where (in which) I was born. 北京是我的出生地。
Is this the reason why (for which) he refused our offer? 这就是他拒绝我们帮助他的理由吗？
2）that代替关系副词
that可以用于表示时间、地点、方式、理由的名词后取代when, where, why和"介词+ which"引导的定语从 句，在口语中that常被省略，例如：
His father died the year (that / when / in which) he was born. 他父亲在他出生那年逝世了。
He is unlikely to find the place (that / where / in which) he lived forty years ago. 他不大可能找到他四十 年前居住过的地方。
判断关系代词与关系副词
方法一： 用关系代词，还是关系副词完全取决于从句中的谓语动词。及物动词后面无宾语，就必须要 求用关系代词；而不及物动词则要求用关系副词。例如：
（错） This is the mountain village where I visited last year.
（错） I will never forget the days when I spent in the countryside.
（对） This is the mountain village (which) I visited last year.
（对） I'll never forget the days (which) I spent in the countryside.
习惯上总把表地点或时间的名词与关系副词 where, when联系在一起。两题错在关系词误用。
方法二： 准确判断先行词在定语从句中的成分(主、谓、宾、定、状)，也能正确选择出关系代词/关系 副词.先行词在从句中作主、定、宾语时，应选择关系代词；先行词在从句中作状语时，应选 择关系副词。
例如：Is this the museum which you visited a few days ago?
Is this the museum where the exhibition was held?
三、定语从句的特殊使用：
（一）定语从句中只能使用that，不用which的情况
1、先行词为anything，everything，all，little，much，nothing，the one, few, any, little，every, no 等不定代词时，只能使用“that”。但用something时，用which或者that均可。例如：
Is there anything that I can do for you? 我能给你做点什么吗?
2、先行词被all，every，no，any，some，little，much，one修饰时，只能使用“that”。例：
Tom told her mother all that had happened.
汤姆把事情的全部情况告诉了他的妈妈。
3、当先行词是形容词最高级或被形容词最高级或序数词修饰修饰时，只能使用“that”。例：
He was the first person that passed the exam. 他是第一个通过考试的人。
This is the most wonderful time that I have ever had. 这是我度过的最美好的时光。
4、先行词被the only，the very，the same，the last修饰时，只能使用“that”。例：
This will be the last chance that he can get. 这将会是他得到的唯一机会。
He is the only person that can help you out. 他是唯一能帮你的人。
5、先行词里同时含有人或物时，只能使用“that”。 例：
They talked of things and persons that they remembered in the school.
他们谈论他们记得的有关学校的事和人。
6、当主句是以who ,which, what开头的特殊疑问句或先行词是who时，只能使用“that”。例：
Which is the book that he bought from the library yesterday?
哪本是你们昨天从图书馆借的书？
7、在there + be 的句型中，句子的主语是先行词，而且又是物。例如：
There are two novels that I want to read. 我要读的有两本小说。
There is no work that can be done now. 没有什么工作现在能做的了。
8、以Here is (are)开头的句子时。 例如：
Here is a film that will move anyone.这是一部将使任何人受感动的电影。
Here are two books that I will buy.这是我要买的两本书。
9、It is (high) time +定语从句中。例如：
It is time that we should have a rest. 我们应该休息了。
It is high time that they started out. 他们该动身了。
10、在双重限定性定语从句中，如果一个从句用who或者which引导时，那么另一个从句用that引导。例如：
He is the student that I have ever see who can jump highest.
他是我曾经看到过跳得最高的学生。
My brother studies in the school which is the most beautiful in our city that isn't far from here.
我的弟弟在我们的城市最美丽的学校读书，并且离这儿不远。
11、当先行词是to be的表语，或关系代词本身是从句的表语时，关系代词应用that。如：
﹡ It's a song that is very popular． ﹡ My hometown is no longer the place that it used to be．
12、与the same（指同一物）连用，构成the same．．．that．．．结构时，关系代词只能用that。如： This is the same museum that you once visited．
注意：the same．．．that．．．结构中的that不能用as代替，
因为the same．．．as．．．与the same．．．that．．．的含义不同。
（二）定语从句中只能使用which，不用that的情况：
1、当主句先行词后有介词时，用which。
﹡ We depend on the land from which we get our food. ﹡ This is the one of which I'm speaking.
﹡ We depend on the land that/which we get our food from.
2、非限定性定语从句中
The Mota, which is four hundred years old, is very famous here.
The sun heats the earth, which is very important to living things.
3、描述句中一般用which。例：
Beijing，which was China's capital for more than 800 years.
4、those +复数名词之后，多用which .例：Shopkeeper want to keep a number of those goods which sell best。
5、先行词本身是that时，用which。
I think that that "that" which that student wrote on the blackboard was wrong.
What's that which looks like a dog?
（三）定语从句中只能使（多）用who，不用that的情况：
1、先行词本身是that，多用who，不用that的情况
1）先行词为anyone，one，ones时；
2）先行词为those，he和people时；
2、如果先行词是those, they, all, persons, people, he, anyone, one时用who
3、当先行词指人并含有较长的后置定语从句或在被分割的定语从句中时。
4、there be句型中，指人用who，指物用that。
四、限定性定语从句与非限定性定语从句：
（一）限定性定语从句
限定性定语从句对被修饰的先行词有限定制约作用，使该词的含义更具体，更明确。限制性定语从句不能被省略，否则句意就不完整，例如：
I met some one who said he knows you.
在引导限定性定语从句时，that有时相当于in which, at which, for which或on which
（1）I like the music for the very reason that(for which) he dislike it.
我出于某种原因喜欢这种音乐，而他恰恰与我相反。
（2）We arrived the day that(on which) they left. 刚好我们到的那天他们走了。
在限定性定语从句中，当关系代词在从句中担任动词宾语时，关系代词可省略。
（1）The book (that/ which) I am reading is very interesting. 我在读的书很有趣。
（2）Is there anything (that) you wanted? 想要什么东西吗?
（二）非限定性定语从句
非限定性定语从句起补充说明作用，缺少也不会影响全句的理解。在非限制性定语从句的前面往往有逗号隔开，如：
（1）The house, which I bought has a lovely garden. 我买的房子带着个漂亮的花园。
（2）This novel, which I have read three times, is very touching.
这本我已经读过三遍的小说很感人。
1.非限定性定语从句可将整个主句作为先行词, 对其进行修饰,这时从句谓语动词要用第三人称单数，例如：
（1）He seems not to have grasped what I meant, which greatly upsets me.
他似乎没抓住我的意思，这使我心烦。
（2）Liquid water changes to vapor, which is called evaporation.
液态水变为蒸汽，这就叫做蒸发。
2◆ 在非限定定语从句中，有时as也可用作关系代词，若as在从句中作主语，其引导的句子可以放在句首，也可以放在句中。例如：
（1）The boy has as much progress as we had expected.
正像我们所预料的那样，这个男孩取得了大的进步。
（2）As everyone knows ,China is a beautiful country with a long history .
每个人都知道，中国是一个有着悠久历史的美丽国家。
3. 在非限定性定语从句中，不能用从句做主语
（1）The boys，who are playing football，are from Class Six. 在踢足球的男孩们是六班的。
（2）I helped an old man，who lost his way，yesterday . 昨天，我帮助了一个迷路的老人。
定语从句中的"as"用法：
1)."as"引导限制性定语从句
a. "such....as"
① He is not such a fool as he looks. ② I have never heard such a story as he tells.
b. "the same ....as" 如：This is the same book as I lost last week.
(区分"the same...as"与"the same....that"：两者都引导定语从句。that从句中说明的名词与the same修饰的名词是同一个。as从句中说明的名词与the same修饰的名词是同一类，而非同一个。举例：①This is the same pen that I lost. 这支钢笔就是我丢的那一支。②This is the same pen as I lost. 这支钢笔跟我丢的那支笔一模一样。)
c. "as...as" 如：As many children as came were given some cakes.
2)."as"引导非限制性定语从句。常用句式:
as is said above 综上所述 as is known to all 众所周知
as is often the case 通常如此 as is reported in the newspaper 如报纸所报道的
as is reported on the Dream Tower website 如三思网所报道的
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