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。

1. 通常要用as的場合

①當從句位于主句前面時，只用 as。如：

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。

②用于such, as, the same后引導限制性定語從句時，只能用as。如：

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

This is the photo which shows my house. 這張照片拍的是我的住宅。

2. 兩者的細微區別

①as引導的非限制性定語從句應與主句在意義上和諧一致，which無此限制。如：

He went abroad, as [which] was expected. 他出國了，這是大家預料到的。

He went abroad, which was unexpected. 他出國了，這讓大家感到很意外。(不用as)

②as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：

The river, which flows through London, is called the Thames. 這條流經倫敦的河叫泰晤士河。(不用as)

③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：

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

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