I never heard such stories as he tells. 我從未聽過他講那樣的故事。(不可用which)

It’s the same story as I heard from her yesterday. 這故事跟我昨天從她那兒聽到的一樣。(不可用which)

2. 引導非限制性定語從句時，有時兩者可互換。如：

I live a long way from work, as [which] you know. 我住得離工作單位很遠，這你是知道的。

但在，在以下情況引導非限制性定語從句時，兩者不可換用：

①當從句位于主句前面時，只用as。如：

As you will find out, I will never let you down. 你將會發現，我決不會讓你失望。

As is known to everybody, the moon travels round the earth once every month. 月球每月繞地球轉一周，這是每個人都清楚的。

②as引導的非限制性定語從句應與主句在意義上和諧一致，而which則無此限制。如：

She married him, as [which] was natural. 她嫁給了他，那是自然的事。

She married him, which was disgraceful. 她嫁給了他，那是很丟人的。(不用as)

③as引導非限制性定語從句時，先行詞通常不能是主句中某個具體的詞，而應是整個句子、整個短語或某個短語推斷出來的概念，而which則無此限制。如：

I once lived in Guangzhou, which is the largest city in south China. 我曾住在廣州，那是中國南方最大的城市。(不用as)

④當as引導非限制性定語從句作主語時，其謂語通常應是連系動詞，而不宜是其他動詞，而which則無此限制。如(www.yygrammar.com)：

She has married again, as [which] seemed natural. 她又結婚了，這似乎很正常。

She has married again, which delighted us. 她又結婚了，這使我們很高興。(不用as)