(3) as 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：

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)

(5) as 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：

Water, which is a clear liquid, has many uses. 水是一種清澈的液體，有許多用途。(不用as)

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)

但是，若先行詞不是某個具體的事物，而是一個抽象的概念，也可以用as。如：

The two brothers were satisfied with this decision, as was agreed beforehand. 兩兄弟對此決定都滿意，這項決定在事前已都得到他們的同意。

(6) 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：

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)

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