(3) as 引导的非限制性定语从句应与主句在意义上和谐一致，which无此限制：
He went abroad, as [which] was expected. 他出国了，这是大家预料到的。
He went abroad, which was unexpected. 他出国了，这让大家感到很意外。(不用as)
(5) as 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：
Water, which is a clear liquid, has many uses. 水是一种清澈的液体，有许多用途。(不用as)
The river, which flows through London, is called the Thames. 这条流经伦敦的河叫泰晤士河。(不用as)
但是，若先行词不是某个具体的事物，而是一个抽象的概念，也可以用as。如：
The two brothers were satisfied with this decision, as was agreed beforehand. 两兄弟对此决定都满意，这项决定在事前已都得到他们的同意。
(6) 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：
She has married again, as [which] seemed natural. 她又结婚了，这似乎很自常。
She has married again, which delighted us. 她又结婚了，这使我们很高兴。(不用as)