定语从句。
用作定语的从句叫做定语从句。引导定语从句的关联词为关系代词或关系副词。关系代词在定语从句中可用作主语、宾语、定语等；关系副词在定语从句中用作状语。
定语从句分类
定语从句有限制性与非限制性之分。
1． 限制性定语从句不可缺少，否则会影响全句的意思；这种定语从句前面一般不用句号。
this is the house jack built.
2． 非限制性定语从句如果缺少了，也不会影响全句的意思，前面一般都会有逗号，非限制性定语从句相当于并列句。注意引导非限定性定语从句的关系代词和关系副词不能省略。that 不引导非限定性定语从句。
he has two sons ,who are doctors.
关系代词引导的定语从句。
用作关联词的关系代词有that,which,who ,whom,whose,as 等。关系代词引导从句，代替先行词，在从句中充当一定句子成分，如主语、宾语、表语、定语等。选择关系代词的时候，需要考查定语从句是限制性还是非限制性，先行词是指人还是指物以及是否具有特殊性，定语从句中缺少哪一种成分，关系代词是否可以省略。
关系代词that
1． 可指人，也可指物，总是用于限制性定语从句中。可在从句中作主语、宾语、表语。
this is the book that you are looking for.
2． that 在限制性从句中作宾语，常常可以省略。
3． 在限定性定语从句中，先行词是物的时候，只用that 不用which的情况如下。
1） 当先行词是不定代词all, the one等时。
do you h**e anything that you want to say for yourself?
all that glitters is not gold.
2) 当先行词是形容词最高级或先行词之前有形容词最高级修饰时。
it is the nicest film that i h**e ever seen.
3) 当先行词是序数词或被序数词修饰的时候。
it is the first film that i h**e ever read
4) 当先行词之前有the only, the very, any,just, few, little, no ,all 修饰的时候。
this is the very bus that i h**e been waiting for.
5) 当先行词即有人也有物的时候。
do you remember the school and the teachers that you visited?
6) 当主句的主语是疑问词who或which 的时候。
which is the book that you need?
who is the boy that was praised ?
7) 有两个定语从句时，其中一个用关系代词which ,另一个用that .
they set up a factory, which produced things that could cause pollution.
8) 当先行词在主句中作表语，而关系代词在从句中也作表语时。
shanghai is no longer he city that it used to be.
关系代词which
1. 一般指物，可在从句中作主语、宾语、定语。也可指前面的一个句子或其部分意义。
the house which stands near a river is our school.
he said that he had never seen her before, which was not true.
2． 当which作介词宾语时，介词可以放在which 之前，也可以放在从句原来的位置上。但在含有介词的固定词组中，介词只能放在从句原来的位置，不能提前，此时不能省略。
this is the book for which you asked.(不可用that)
this is the book that/which i h**e been looking for.( 不能提前for)
3． 只用which 不用that 引导定语从句的情况。
1） 在非限制性定语从句中，只用which, 不用that .
he was late again, which made me very angry.
2) 当动词短语中的介词提前时，只能用which,不用that .
this is a house in which he once lived.
关系代词who, whom, whose 的用法。
1． 当先行词指人，在从句中作主语时，用who/that ,不可省略。
she is the girl who lives/that lives next door.
2. 当定语从句中作宾语时，用whom/that /who ，可以省略。当whom作介词宾语时，介词可以放在whom 之前，也可以放在从句原来的位置上。
但在含有介词的固定词组中，介词只能放在从句原来的位置，不能提前，此时whom 不省略。
that’s the girl whom/who/that i teach.
3.在定语从句中作定语时，用whose ,先行词可以是人或物，不可省略。当先行词是人时，whose= of whom, 当先行词是物时，whose=of which ,但需要在被修饰的名词之前加the.
this is the house whose window broke last night.
=this is the house of which the window broke last night.
= this is the house the window of which broke last night.
关系代词as
1. 限制性定语从句中，用在such, same, as或so的后面，可作主语、宾语和表语。如：
he can write so interesting a story as moves us to tears.
we h**e found such materials as are used in their factory.
he is not the same man as he was.
1）注意such …as 引导的定语从句和 such …that 引导的状语从句的区别。
he is such a clever boy as everyone likes. (定语从句，as 在从句中作成分)
he is such a clever boy that everyone likes him. (状语从句，that 在从句中不作成分)
2) such as ….such 为代词，“这样的人或事物”。 as 在从句中作成分，修饰先行词such.
this book is not such as i expect.
2. 非限制性定语从句中，这时先行词为主句的全部或后面部分，as 有“正如……”之意。
as you know, i am fond of music.
3. as 和which 的区别。
1) as引导的非限制定语从句可指置于主句前，主句末，也可插入主句中。而which 引导的非限定性定语从句只能置于主句之后，相同的是两者都可替代主句的整个内容，而不是主句的某一个词。as 可用于以下习惯用语中：
as is known to everybody, the moon tr**els around the earth once every month.(不能用which,因为which引导的非限制性定语从句只能置于主句末)
2) as 用于习惯用语中： as you can see, as is well-known, as is known to all, as we had expected, as has been said before, as was said earlier, as i remember , as often happens, as is often the case 等。
3）当非限制性定语从句时否定句或表示否定含义时只能用which.
he came here early, which was un expected.
4) 当as 在从句中作主语时， 后面常接动词的被动语态，如be known, be said, be reported,等。如果从句的动词时主动语态，一般用which.
she has been absent again, as is expected.
he has made great progress, which made me happy.
介词+关系代词。
1.“介词+关系代词”引导定语从句时，关系代词只能用which, whom, whose 且不能省略，其中whose 与所修饰的名词一起作介词的宾语。
the driver was the man from whose room the thief had stolen the bag.
2. 介词的选择需要考虑“动词+介词+名词（先行词）”等搭配。如：
the car on which he spent 40,000 yuan is made in nanjing.(spend money on sth)
3. 关系代词前的介词可以是复合介词。即“复合介词短语+关系代词which.”这种结构引导的定语从句常与先行词用逗号分开，定语从句常用倒装语序。
she g**e a friendly smile to the worker, in front of whom sat a little cat.
4. 介词前可用名词、代词、数词、形容词最高级，也可转换成“介词+关系代词+（the）+名词/代词/数词/形容词最高级”形式。如：
she has two sons, neither of whom is tall.(复合句)
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