定语从句之专题三：as引导定语从句（系统讲解内在原理）

（2）which作宾语时，有时可以替换为as。试对比：

●He is from the south, which we can know from his accent.（不能插入it）

●He is from the south, as we can know (it) from his accent.（有无it都可以）

（3）which作表语时，有时可以替换为as。试对比：

●He seemed a foreigner, which in fact he was.（不能插入a foreigner）

●He seemed a foreigner, as in fact he was (a foreigner).（有无a foreigner都可以。因为，在状语从句中，承前省略了a foreigner。） 

总之，as是连词，which是代词，两者截然不同。二者能换用的情况，纯属巧合，并不能说明它们词性相同。打个比方，一个马群和一个牛群，偶尔在同一地方吃草，但是，马还是马牛还是牛。

3.常见的as引导评述性状语从句。见下表，

分类

举例

主系表

as (it) is often the case，as (it) is natural，as (it) seems likely

主系+v-ed

as (it) was pointed，as (it) is mentioned/repeated/said，

as (it) have been announced，as (it) has been said before

主谓

as (it) often happens，as (it) appears

as I remember/understand (it)，as we had expected (it)，

as I have said (it)，as we see/know (it)，as everyone knows (it)

注：关于状语从句中的省略，可以参看本章第七节以及第十二章。

4. 注意as/it/what的区别。如，

●What is known to us all is that China has the largest population in the world.

（what引导主语从句，that引导表语从句）

●It is known to us all that China has the largest population in the world.

（it作形式主语，真实主语是后边的that从句）

●As is known to us all, China has the largest population in the world.

（as引导评述性状语从句）