非限定性定语从句中which和as的区别

as与which引导非限制性定语从句的区别：
这类定语从句只能由 which 或 as 引导。两者之间的区别在于：
1. which 引导的定语从句只能位于主句后，但as引导的从句可位于主句前、中、后。如：
David, as you know, is a photograopher. 戴维是个摄影师，你是知道的。(不可用which)
Li Ming is late, as is often the case. 李明迟到了，这事是经常发生的。(不可用which)
2. as在定语从句中作主语时，从句谓语通常要有be或别的系动词，但which不受此限制。如：
Li Ming was late, which (=and this) made Mr. Zhang very angry. 李明迟到了，这件事使张先生非常生气。(不可用as)
3. as 引导的定语从句只表示一个众所周知或意料之中的事，但 which 不受此限制。如：
He married her, which was unexpected. 他和她结婚了，这是出乎意料的。(不可用as)
4. as有“正如”之意，而which则意为“这(件事，一点)”，可用and this/that/it来代替。
5. as多用于固定搭配中：as is often the case(这是常有的事)，as was expected(不出所料)，as often happens(正如经常发生的那样)，as is known to all(众所周知)，as has been said before(如上所述)，as is mentioned above(正如上面所提到的)等。
注：which 代表整个句子，还可用于in which case, at which point, on which occasion等。如：
I may have to work late, in which case I’ll telephone you. 我可能得晚点下班，那样我会给你打电话的。

as一般放在句首， which在句中。

As we know, smoking is harmful to one's health.

The sun heats the earth, which is very important to us.