定语从句。
一、定义及特点：在复合句中用来修饰某一名词或代词的从句叫定语从句。其特点是定语从句的引导词是先行词的替身，既起联系作用，同时又充当定语从句的一个成分，所以掌握定语从句的关键在于：
弄清楚其先行词的所指、所作成分和可否省略。
二、分类及区别：定语从句分为限制性定语从句和非限制性定语从句两种。它们的区别有四点：
1、从重要性上看：限制性定语从句在整个句子中比较重要，省去后会影响整个句子意思的表达；而非限制性定语从句则反之。 2、从句子形式上看：
限制性定语从句的引导词前面无逗号，而非限制性定语从句的引导词前面有逗号。 3、从引导词上看：that，why可以引导限制性定语从句，但却不可以引导非限制性定语从句。
4、从引导词可否省略上看：在限制性定语从句中作宾语的引导词可以省略，而在非限制性定语从句中的引导词无论作什么成分都不可以省略。
三、用法（详解）
一、限制性定语从句的用法：
一）关系代词引导的定语从句。
1、关系代词that，which，who，whom，whose，as引导的定语从句的用法。
⑴that可以指人，也可以指事物，在定语从句中作主语或宾语（作宾语时可以省略）。如：who is the man that is reading a news***** there？
（作主语指人）
the girl（that）we saw yesterday is jim's sister.（作宾语指事物）
a dolphin is an animal that lives in the sea.（作主语指事物）
the moon cakes that mother cooked taste nice.（作宾语指事物）
⑵which指事物，在定语从句中作主语或宾语（作宾语时可以省略）。
如：the silk which is produced in suzhou sells well.（作主语指事物）
the song（which）the singer sang were very popular.（作宾语指事物）
who，whom指人：who，whom在定语从句中分别作主语或宾语（who也可以作宾语，who/whom作宾语时可以省略）；whose可以指人（= of whom）或事物（=of which）不可以省略）。如：
the student who sits on my left is an american.（作主语指人）
the person（who/whom）you just talked to is our headmaster.（作宾语指人）
i know the woman whose daughter studies abroad.（作定语指人）
as指物，常用于such…as，the same…as，as…as结构中，在定语从句中作宾语、表语、定语和状语，不可以省略。如：
this is not such a book as i expected.（作宾语指物）
it is the same place as it used to be.（作表语指物）
i like the same book as you do.（作定语指物）
i shall do it in the same way as you did.（作状语指物）
二）关系副词引导的定语从句。
1、关系副词when，where，why引导的限制性定语从句的用法。
when指时间，在定语从句中作时间状语，不可以省略。如：
we still remember the years when/during which we studied together.
where指地点，在定语从句中作地点状语，不可以省略。如：
this is the place where/in which he lives.
why指原因，在定语从句中作原因状语，不可以省略。如：
do you know the reason why/for which i was late for class？
根据定语从句先行词的所指（指人/物/时间/地点/原因）、先行词的所做成分、及其可否省略，三个因素来综合确定定语从句的引导词。
一、 四个引导词。
1.只用that引导定语从句的情况：
当先行词既有人又有物时，只能用that。如：
they talked about the things and persons that they remembered in the school.
当先行词是数词或all，much，little, everything，anything，nothing，none，the one等指物的不定代词时，只能用that。如：
yesterday i caught two fish and put them in a basin of water.
now you can see the two that are still alive.
i had to remember everything that the teacher taught me.
注意：当先行词是something时，关系代词用that/which都可以；当先行词是someone，anyone，everyone，somebody，anybody，nobody，everybody，all，those，one（s）指人时，关系代词用that，who，whom都可以。
③当先行词被all，much，some，any，（a）few，(a) little，no, none, no one等修饰时，只能用that。如：
i h**e done all the things that he told us to do. he has no books that i need.
④当先行词是序数词/形容词最高级或被序数词/形容词最高级修饰时，只能用that。如：
he was the first（person）that got to the top of the mount qomolangma.
he worked out the most difficult problem that i had seen.
i like the tallest（person）that was asked to come here.
⑤当先行词被the very，the only，the last修饰时，只能用that。如：
it is the very book that i am looking for.
⑥当主句主语为what, who，which开头的特殊疑问句时，只能用that。如：
who is the boy that was here just now？
which is the bike that you lost？
⑦ 主句是there/here be结构时，当主语指物时，修饰其主语的定语从句用that引导；同时，定语从句部分是there be…句型时，也只能用that引导。如：
there is still a seat that is free.
he asks for a book that there is on the subject.
⑧当句子中有两个以上从句，其中一个已经用了which时，只能用that。如：
edison built up a factory which produced things that had never been seen before.
⑨当先行词是主句的表语而引导词又在定语从句中作表语时，只能用that。如：
china is no longer the country that it used to be.
2.只用which引导定语从句的情况：
①作介词的宾语且介词提前的时候（此时which不可省略）。如：this is the house of which the windows face south.
②引导非限制性定语从句的时候（此时which不可省略）。如：his dog，which was very old，became ill.
③一个句子有两个定语从句时，为了避免重复，一个用that引导，另一个用which引导。如：he built up a factory that produce things which had never been seen before.
④先行词为that/those时。如：
what was that which we saw last night?
⑤先行词后面有插入语时。如：
the book, i thought, which you lent me yesterday is very expensive.
3. as引导定语从句的情况：
as可以引导限制性定语从句，也可以引导非限制性定语从句，as在从句中作主语、表语和宾语。如：we h**e found such materials as are used in their factory.
(as作主语)
these houses are sold at such a low price as people expected. (as作宾语)
he is not the same man as he was. (as作表语)
①as引导限制性定语从句的情况：
a.当先行词被such, so, as修饰的时候，常用as引导定语从句。此时应注意与such/so…that
引导的结果状语从句区别开来。如：
he is such a good person as i always dream of ****** friends with. （as引导的定语从句）
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