关于非限制性定语从句

在引导非限制性定语从句时，若从句谓语动词是be动词，作为主语或表语的引导词可以用as或which. He said he could catch a bird in the sky,as(which) was a lie. 若从句谓语动词是非be动词，做主语的引导词必须用which.做宾语的引导词用which或as均可。He decided to help me,which made me excited.
He decided to help me,which(as) I expected.若定语从句前移，必用as.例如As we know,he is excellent.

告诉你吧，as和which都可以引导定语从句。定语从句的关键点是抓“先行词”

as+从句 这个as引导非限制行定语从句，as指代前面主句的整个“句子”。
He is an englishman,as I know from his accent.

which+从句 这个which指代前面主句的"一个成分"。
the girl which you saw is my girlfriend.这个which指代the girl.

当然 as也可以引导 限制性定语从句，主要在这样的短语中such...as the same...as

this is the same pen as I lost last week.
there is no such people as he is.

3) 非限制性定语从句还能将整个主句作为先行词, 对其进行修饰, 这时从句谓语动词要用第三人称单数，例如：
He seems not to have grasped what I meant, which greatly upsets me. 他似乎没抓住我的意思，这使我心烦。
Liquid water changes to vapor, which is called evaporation. 液态水变为蒸汽，这就叫做蒸发。

as, which 非限定性定语从句