but，as，than作关系代词引导定语从句
but既可指人，也可指物，常在定语从句中充当主语。

but本身含"否定"的意思，其作用相当于"that /which /who．．．not"。

它前面的主句通常有"否定"的词（如：no，not， little，few，hardly等）。

but引导的肯定定语从句和否定的主句连用，达到强烈肯定的修饰效果。

例如： There is no mother but loves her own children．（=There is no mother that/who does not love her own children．）没有不爱自己孩子的母亲。

as既可指人，也可指物，常在限制性定语从句中充当主、宾、表语。

as引导的定语从句的先行词前面常有as /such /the same等修饰词。

例如：①He is as brave a man as ever lived．他是世界上最勇敢的人。

②Don’t read such books as are not worth reading．不要读那些不值得读的书。

as作关系代词，还可用来引导非限制性定语从句，as引导的定语从句可放在句首、句中或句末。

例如：① Taiwan，as we all know，is an inseparable part of China．②As we all know，Taiwan is an inseparable part of China．③Taiwan is an inseparable part of China， as we all know．
than既可指人，也可指物，可作关系代词来引导定语从句。

than前通常有表比较的词。

例如：Fewer friends than we had expected came to our evening party．来参加晚会的朋友比我们预料的还要少。

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｡。