as引导非限制性定语从句

as引导非限制性定语从句：as可以放在主句前后，也可插入主句之中、在非限制性从句中as而后的be动词可以省略、as做主语时，谓语常用系动词，如be,seem,become等，一般不用其他行为动词、as常用“正如"含义。

as可以放在主句前后，也可插入主句之中：

As is known to all, fish can' t live without water.

众所周知，鱼离不开水。

在非限制性从句中as而后的be动词可以省略：

She told me she won the match, which was a lie.

她告诉我她赢了比赛，这是谎言。

as做主语时，谓语常用系动词，如be,seem,become等，一般不用其他行为动词：

He saw the girl, which delighted him.

他看见了那个女孩，这使他很高兴。

as常用“正如"含义，常用的结构有as we know(众所周知）；as often happens(正如常发生的那样）;as is often the case（情况常常如此）;as we all can see（正如我们看到的），这些结构常放句首，偶尔也可以放句中或句末。

As is known to all, China is a developing country.

众所周知，中国是一个发展中国家。

as引导限定性定语从句：

so/such …… as ...... 结构

例句：Such students as work hard are meant to be somebody in the future. (as在句中充当主语) 如此努力的学生将来一定很有出息。

the same ...... as …… 结构

例句：We are all faced with the same environmental problem as is caused by the serious air pollution.(as在句中充当主语)

我们都面临相同的由于空气污染所造成的环境问题。

as ...... as …… 结构

例句：As many students as were absent from class were punished by their headmaster.(后面的as在定语从句中充当主语)

所有逃课的学生都被校长处罚了。