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。
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