這類定語從句只能由which或as引導(dǎo)。兩者之間的區(qū)別在于：

1. which引導(dǎo)的定語從句只能位于主句后，但as引導(dǎo)的從句可位于主句前、中、后。如：

David, as you know, is a photograopher. 戴維是個(gè)攝影師，你是知道的。(不可用which)

Li Ming is late, as is often the case. 李明遲到了，這事是經(jīng)常發(fā)生的。(不可用which)

2. as在定語從句中作主語時(shí)，從句謂語通常要有be或別的系動(dòng)詞，但which不受此限制。如：

Li Ming was late, which (=and this) made Mr. Zhang very angry. 李明遲到了，這件事使張先生非常生氣。(不可用as)

3. as引導(dǎo)的定語從句只表示一個(gè)眾所周知或意料之中的事，但which不受此限制。如： He married her, which was unexpected. 他和她結(jié)婚了，這是出乎意料的。(不可用as)

4. as有“正如”之意，而which則意為“這(件事，一點(diǎn))”，可用and this/that/it來代替。

5. as多用于固定搭配中：as is often the case(這是常有的事)，as was expected(不出所料)，as often happens(正如經(jīng)常發(fā)生的那樣)，as is known to all(眾所周知)，as has been said before(如上所述)，as is mentioned above(正如上面所提到的)等。

注：which代表整個(gè)句子，還可用于in which case, at which point, on which occasion等。如：

I may have to work late, in which case I’ll telephone you. 我可能得晚點(diǎn)下班，那樣我會(huì)給你打電話的。