关系代词
定语从句在句子中充当定语功能的从句，修饰一个名词或代词，有时也可修饰一个句子。由于定语从句由关系词引导，故又常称作关系代词。所谓关系词是指这类词在先行词和从句之间发挥着一定的关系作用，即：既引导定语从句，又代替先行词在从句中充当着某个成分。
各个关系代词的代替功能以及它们在从句中的作用见下表：
代替人代替功能 在从句中的作用 主语 宾语 Who Whom Whose 定语 (=of whom) Which Which Whose (=of which) that that 代替人 代替物 或物 例如：Where is the girl who (或that) was injured in the accident?(关系代词作主语)
The man whom (或that)you saw just now is our manager.(关
系代词作宾语)
He found the watch which (或that) he had lost.
[注]：有时，关系代词that还可代替在定语从句中担任表语的先
行词。例如：
She is no longer the girl that she was in her childhood.
1）
关系代词的省略
（1） 在口语中以及非正式文体中，在定语从句里充当宾语作用的关系代词常常省去。
例如：Have you found the book (that) you want?
（2） 当关系代词紧跟介词，作介词宾语时，不可以用that，而只能用whom或which并且不可省略。但是当介词位于定语从句句尾时，作为介词宾语的关系代词则仍可用that，也可以省去。试比较：
The book from which (不用that) a got a lot of useful information was written by a famous physicist.(不可省略)
The book (that 或which) I a got a lot of useful information from was written by a famous physicist.(可以省略) 2）以下两种情况要求使用关系代词that
（1）当先行词是all, everything, something, nothing等不定代
词，或被first，but，only, few, much, some, any, no以及形容词最高级等词修饰时，应用关系代词that，不用which。例如:
I am interested in all that you have told me. He asked for the best book that was on the subject.
（2）当并列的两个先行词分别表示人和物时，要用关系代
词that引导定语从句。例如：
We were deeply impressed by the teachers and schools that we had visited there.
摘自：《实用大学英语语法》 东南大学出版社 主编：李俊峰
关系代词一方面引导定语从句，修饰其前面的名词，代词或从句；另一方面又在从句中担任一个成分，如主语、宾语、表语和定语。
There was a man who called to see you when you were out.(who指a man，并作从句的主语)
【注】：指人时，who比that用得多一些。在最高级的定语从句中，that用得多，优于who：
New was one of the greatest men that ever lived. 指事物时，that比which用得多一些：
There are lots of things(that)I need to do before I leave tonight. 定语从句有限定性和非限定性两种。
在限定性定语从句中，如果关系代词作宾语，则常被省略，特别是在口语中：
There is a method (which) we must master.
如果关系代词前面紧挨着介词则不能省略，而且不能用that，只能用which：
The photos at which you were looking were all taken by my brother. 在非限定性定语从句中，只能用who, whom代表人；用which代替事物：
I discussed it with my brother, who is a lawer.
He said he had lost the book, which (=but this )was untrue.
摘自：征服基础英语语法 北京大学出版社 主编：刘小梅
限定非限定性 性 指物 指人 主 格 宾 whom 格 属 格 e whosof of which/whose which/whose which that/which who/which that that/which 指物 限定性 关系代词有，宾格和属格（所有格）之分，并有指人与指物之分。that的用法最广，that 可指人也可指物。见表：
that 的用法
1）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.
2) 只能用that作为定语从句的关系代词的情况 a) 在there be 句型中，只用that，不用which。
b) 在不定代词，如：anything, nothing, the one, all, much, few, any, little等作先行词时，只用that，不用which。
c) 先行词有the only, the very.（恰恰，正好）.any. few. little. no. all 修饰时，只用that。
d) 先行词由序数词、数词、形容词最高级修饰时，只用that。 e) 先行词既有人，又有物时。
Finally, the thief handed everything that he had stolen to the police. 那贼最终把偷的全部东西交给了。
不能用Which的几种情况
1.当先行词为all, little, much, few, everything, none 等不定代词时关系代词用that而不用which.
There are few books that you can read in this book store. 2.当先行词既有人又有物时,关系代词用that, 不用which. He asked about the factories and workers that he had visited. 3.当先行词有形容词最高级修饰时,关系代词用that, 而不用which.
Guilin is one of the most beautiful cities that I have ever visited. 4.当先行词有序数词修饰时,关系代词用that, 不用which. The first sight that was caught at the Great Wall has made a lasting impression on him.
5.当先行词被the only, the very, the same, the right等修饰时,关系代词用that,不用which.
It is the only book that he bought himself.
6.当先行词被all, every, any, much, little, few, no等修饰时,关系代词用that,不用which.
There is no clothes that fit you here.
7.当主句是以which开头的特殊问句时,定语从句关系代词一般用that而不用which.
Which of the books that had pictures was worth reading? 8.在强调句型\" It is...that ...\" 中,只用that,不用which. It is in this room that he was born twenty years ago.
9.在\" such (the same)...as ...\" 句型中,关系代词要用as, 而不用which.
We need such materials as (not which) can bear high temperature.
10.表示\" 正如... 那样 \正像...\" 之意时, 用as, 不用which. Mary was late again, as had been expected.
11：As 引导的定语从句可以放在主句前面, 也可以放在主句后面或主句中间,而由 which引导的定语从句只能放在主句后面。
As we all know, oceans cover more than 70% of the earth. used to/be used to的分别
used to + do：\"过去常常\"表示过去习惯性的动作或状态，但如今已不存在。
Mother used not to be so forgetful.
be used to + doing： 对……已感到习惯，或\"习惯于\"，to是介词，后需加名词或动名词。
Scarf is used to taking a walk.(现在习惯于散步)
used to 的用法
（否定式简写为usedn't） 过去经常，以前常常 This used to be a shabby house. （此房年久失修）
used to, would这两个词语都可以表示过去常做某事，有时可以换用。 used to do 强调过去习惯性的行为或状态，但是现在没有这种行为或状态了。因此，这个短语的内涵是今昔对比。
不可省略和可以省略
1.关系代词which ,whom , who , that在定语从句中作宾语时可以省略。
Here is the man (who/that/whom)you're been looking for. 以下情况不能省略：
（1）在介词+whom与介词+which中，whom, which不能省略。 That is the headmaster with whom my father was talking just now.
（2）在非性定语从句中，作宾语的关系代词也不能省略。 The elephant is like a spear ,as anybody can see.
（3）在the same...as ,such...as, as...as ,the same...that中，as, that即使作宾语，也不能省略I have bought the same bicycle as you
have (bought). (指同类用as)
This is the same book that I read the day before yesterday. (that指同一个)
（4）当由and ,but, or等连词连接两个或两个以上的定语从句修饰一个先行词时，第一个关系代词可省，但第二，第三个不可省略。
This is the book (which) I read yesterday and which I find very interesting.
2.当that在从句中作补语时可以省略。 She is all (that) a teacher should be.
3.在there be 结构中出现定语从句，或在定语从句中出现there be 时，用作主语的关系代词也可省略。
This is the best dictionary (that) there is in the library. 4.状语的省略
（1）当先行词是reason，而且定语从句中作原因状语时，关系代词可用why ,that,也可以省略。
That is the reason (why) I did it.
（2）当先行词是way,且在定语从句中作方式状语时，关系代词可用in which, that,也可以省略。
I don't know the way that/ which leads to the top of the mountains.
(3)当先行词是time时，关系代词可用when, that 或省略。
I don't know the exact time (when/that) the sports meet will take place.
注意：关系代词作宾语有时也不能省略。
摘自：百度百科
