关系代词：
用来引导定语从句。它代表先行词，同时在从句中作一定的句子成分。
关系代词有主格，宾格和属格之分，并有指人与指物之分，它在先行词和定语之间起纽带作用。
主要的关系代词有who,whom,whose,which和that。
①who指人，在定语从句中作主语或宾语；
whom指人，在定语从句中作宾语；
whose在定语 从句中作定语，其先行词既可以是人也可以是物。
例如：The teacher who/whom you want to see has come.
②which指物，在定语从句中作主语或宾语。 
例如：Here is the coat which will be made to you.
③that多指物，有时也指人，在定语从句中作主语或宾语。
例如：I have eaten bread that/which he gave to me.

关系代词的基本用法：
（1）引导定语从句
关系代词代表先行词，同时在从句中作一定的句子成分，例如：
The girl whom I spoke to is my cousin. 跟我讲话的姑娘是我表妹。
（该句中whom既代表先行词the girl，又在从句中作介词to的宾语。）
This is the pencil whose point is broken.这就是那个折了尖的铅笔。
(whose 指物，在限定性定语从句中作定语)
He came back for the book which he had forgotten.他重新回来都忘记了他的那本书。
（which指物，在限定性定语从句中作宾语，可以省略）
（2）which为其先行词。例如：
He said he saw me there, which was a lie.他说在那儿看到了我，纯属谎言。
（3）其他用法
关系代词that在从句中作宾语或表语时可省略， 例如：
I've forgotten much of the Latin I once knew.我过去懂拉丁语，现在大都忘了。
He's changed. He's not the man he was.他变化很大，已不是过去的他了。
注释：关系代词在从句中作宾语时可省略，that在从句中作表语时也可省略。
非限定性定语从句中，不能用that作关系代词。

关系代词的语法注意:
1.常用that 不用which的情况:
①先行词为all, anything, everything, nothing, something等时。
I will do all (that) I can to help you.
②先行词被序数词、形容词最高级或the only, the very等所修饰时。
He was the only person that joined the army in his village that year.
③先行词既有人,又有物时
They talked about the people and the things (that)they remembered in the school.
④当主句是以who 或which开头的疑问句时, 定语从句用that 引导, 以避免重复。
Who is the boy that was here just now?
2. 若被修饰的名词充当从句介词宾语时
介词可提前于引导词之前,此时的引导词只能用whom 或which.
The pencil with which he was writing broke.
3. 当先行词为way时
其后的定语从句用in which或 that引导,也可不用引导词。
I don’t like the way (in which/that) he talks to me.

used to/be used to的分别：
①used 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.
②be 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)
③used 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。

关系代词的用法:
一、不用that的情况
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.
介词后不能用We depend on the land from which we get our food.

二、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.先行词为序数词、数词、形容词最高级时，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警察。

三、不能用关系代词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 "，" 正象..." 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.

关系代词可以省略的情况:
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.
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。

关系代词不能省略情况：
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.在非限制性定语从句中，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.

关系代词格式：
关系代词有主格，宾格和属格之分，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：

	限定性 指人	非限定性 指物	限定性 指人或指物
主格	who	which	that
宾格	whom	that	that
属格	whose	of  which whose	of  which whose

特殊关系代词的用法：
关系词通常是用来引导定语从句的,但as,than和but这三个词却与which, who, that, where, when和why等典型的关系词不同。
三者本身具有常见的字面意义,前面有名词作先行词,但后面的定语从句却不像典型的定语从句。
这些既像连接词又像关系词的词,我们就姑且称它们为准关系词。
一、 as作为准关系代词出现在the same ... as, such ... as, 和as ... as等结构中。
这时, as前面通常要有名词;
as后面的从句意义上不像是通常的定语从句;
as需要在从句中充当一定的句子成分。
1. the same ... as
        1) as在从句中作主语
　　She is the same person as came here last Sunday. 她就是上星期天来这儿的那个人。
　　2) as在从句中作宾语
　　This is the same watch as I lost while I was doing the sightseeing in Shanghai three months ago. 这块表跟我三个月前在上海观光时丢失的那块一样。
　　3) as在从句中作be动词的表语
　　China is not the same country as it used to be. 中国已今非昔比。
　　4) as在从句中作行为方式状语
　　You should learn to do housework in the same way as your mother does it. 你应该学会像你母亲那样做家务活。
2. such ... as
　　1) as在从句中作主语
　　Let children read such books as tend to make them better and brighter. 让孩子看那些容易让他们变得更好更聪明的书。
　　2) as在从句中作宾语
　　This is not such an interesting book as you just talked about. 这不是你们刚才谈到的那种有趣的书。
　　3) as在从句中作be动词的表语
　　Such a bad man as he is will be punished. 像他这样的坏人一定会受到惩罚。
3. as ... as
　　1) 第二个as在从句中作主语
　　He has as much money as is required. 他要多少钱就有多少钱。
　　2) 第二个as在从句中作宾语
　　I have as many friends as you (do). 我的朋友和你的朋友一样多。
　　3) 第二个as在从句中作be动词的表语
　　As is known to all, Taiwan is one of the Chinese provinces。众所周知,台湾是中国的一个省。

二、than作为准关系词 出现在more ... than, less ... than, fewer ... than等结构中。这时, than前面要有名词; than本身有比较意义,而且需要在从句中充当一定的句子成分。
　　1. than在从句中作主语
　　Take it easy, I have more money than is needed. 放心，我的钱比所需要的还要多。
　　2. than在从句中作宾语
　　I have more/fewer books than you (do). 我的书比你的多/少。
　　3. than在从句中作be动词的表语
　　He is a better boy than you (are). 他是一个比你更好的男孩。
三、but作为准关系词 本身含有否定意义,它前面的主句还有另一个否定意义。这时but在意义上相当于“who ... not ...”或“which/that ... not ...”。
　　1.定语从句中有be动词,并将其改为含but的复合句时, be动词后面不能再有not
　　There is no one who is not conccrned about his future. (= There is no one but is concerned about his future.) 没有人不关心自己的未来?
　　2. 定语从句中有can, will, shall, must, may, should, ought to等助动词或情态动词,在将其改为含but的复合句时,这些助动词或情态动词后面不能再有not
　　There is nothing that she can’t do. (= There is nothing but she can do.) 没有她做不到的事?
　　3. 定语从句中有助动词do, does, did, 在将其改为含but的复合句时,应去掉do, does, did, 还原其后动词的时态及人称变化
　　There is no one who doesn’t wish to make great achievements. (= There is no one but wishes to make great achievements.) 谁都希望自己将来能有所成就。
四、which的先行词：关系代词which的先行词可以是一个句子
　　He said he saw me there, which was a lie。他说在那儿看到了我，纯属谎言。
说明：关系代词that在从句中作宾语或表语时可省略
　　I've forgotten much of the Latin I once knew。 我过去懂拉丁语，现在大都忘了。