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I 定语从句
1.先行词为all， anything， something， nothing， everything， much， little， none等不定代词时，关系代词一般只用that，不用which。在大多数情况下that可以省略.
Please tell me everything (that) you know about the matter.
That’s all (that) we can do at the moment.
2.as引出的限制性定语从句
在such … as的结构中as可作关系代词，引出限制性定语从句。有时和same连用，在从句中可作主语、宾语或表语等。
Such people as were recommended by him were reliable.
I’ve never seen such a talented young man as he is.
I have the same trouble as you (have)。
3.as引出的非限制性定语从句
as可作关系代词引出非限制性定语从句，代替整个主句，通常译为“(正)如…一样”，“(正)象…一样”等。as引导的从句一般用逗号与主句隔开，可以位于主句的前面、中间或后面.
I live a long way from work， as you know.
She did not， as her friend had feared， break down.
As is generally accepted， economic growth is determined by the smooth development of production.
4.分隔式定语从句
定语从句一般紧跟在先行词之后，但有时会被其他句子成分与先行词隔开，从而构成分隔式定语从句。
The days are gone when power politics worked.
A new teacher will come tomorrow who will teach you French.
5.介词+关系代词(which/whom等)引出的定语从句
如果关系代词(which/whom等)在定语从句中作介词的宾语，那么这个介词可以提到从句前，构成“介词+关系代词(which/whom等)+定语从句”。在这种结构中关系代词不可以用that。
This is a subject about which we might argue for a long while.
The four travelers with whom I shared the room were pleasant people.
6.关系代词在限制性定语从句中常可以省略，一般有以下几种情况：
1) 关系代词在从句中作直接宾语时，在大多数情况下可以省略。
This is something (that) you must always keep in mind.
The man (whom) you just met is our manager.
关系代词作介词宾语时，介词位于句尾时关系代词可以省略;介词位于关系代词之前时则关系代词不能省略。
This is the room (which) Churchill was born in.
This is the room in which Churchill was born.(which不可省略)
2) 关系代词作主语时一般不能省略，但如果关系代词在由there … be存在句构成的定语从句中作主语时常可省略。
This is the only book (that) there is on this subject.
The old professor made full use of the time (that) there was left to him to continue his research.
3) 在way后面的定语从句中in which或that通常省略。
That’s the way (that/in which) I look at it.
I don’t like the way (that/in which) you laugh at her.
7.非限制性定语从句
非限制性定语从句修饰先行词或整个主句，起补充说明的作用，如果省略掉，原句句意仍保持完整。非限制性定语从句要用逗号与主句隔开，其引导词不能用that。非限制性定语从句属于正式语体。经常考到的非限制性定语从句有以下三种。
1)由which、as引导的非限制性定语从句。which、as代表整个主句。
He said that he had never seen her before， which was not true.
China is still a developing country， which is known to all of us.
2)由 “介词+关系代词”引导的非限制性定语从句。
He sent her a letter， in which he said that he was sorry for what he had done to her.
He failed to pass the exam， because of which his parents scolded him.
4) 由 “数词、代词或名词+of +关系代词”引导的非限制性定语从句。
He had three sons， one of whom was my son’s classmate.
There are about twenty students in this course， most of whom are freshmen.
They are two different words， the spellings of which are easily confused.