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。