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。