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 you know. 我住得离工作单位很远，这你是知道的。
1. 通常要用as的场合
①当从句位于主句前面时，只用 as。如：
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。
②用于such, as, the same后引导限制性定语从句时，只能用as。如：
I never heard such stories as he tells. 我从未听过他讲那样的故事。
This is the photo which shows my house. 这张照片拍的是我的住宅。
2. 两者的细微区别
①as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。如：
He went abroad, as 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)
②as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：
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)
③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：
She has married again, as 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)
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