I never heard such stories as he tells. 我从未听过他讲那样的故事。(不可用which)

It’s the same story as I heard from her yesterday. 这故事跟我昨天从她那儿听到的一样。(不可用which)

2. 引导非限制性定语从句时，有时两者可互换。如：

I live a long way from work, as [which] you know. 我住得离工作单位很远，这你是知道的。

但在，在以下情况引导非限制性定语从句时，两者不可换用：

①当从句位于主句前面时，只用as。如：

As you will find out, I will never let you down. 你将会发现，我决不会让你失望。

As is known to everybody, the moon travels round the earth once every month. 月球每月绕地球转一周，这是每个人都清楚的。

②as引导的非限制性定语从句应与主句在意义上和谐一致，而which则无此限制。如：

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

She married him, which was disgraceful. 她嫁给了他，那是很丢人的。(不用as)

③as引导非限制性定语从句时，先行词通常不能是主句中某个具体的词，而应是整个句子、整个短语或某个短语推断出来的概念，而which则无此限制。如：

I once lived in Guangzhou, which is the largest city in south China. 我曾住在广州，那是中国南方最大的城市。(不用as)

④当as引导非限制性定语从句作主语时，其谓语通常应是连系动词，而不宜是其他动词，而which则无此限制。如(www.yygrammar.com)：

She has married again, as [which] seemed natural. 她又结婚了，这似乎很正常。

She has married again, which delighted us. 她又结婚了，这使我们很高兴。(不用as)