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.

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