在非限制性定语从句中 当主句是否定句时,只能用as

在非限制性定语从句中,which和as引导定语从句时的区别
a.which引导的从句不能放在主句前,而as引导的从句则既可以放在主句前,也可以放在主句后.例如：
As (正确) is often the case,he is absent.
Which (错误)
He saw the girl,as (正确) He had hoped.
Which （正确）
b.用能做关系代词,as和which都可以指代一个句子,但as有“正如”、“就像”之意,which则没有此含义.
He is a foreigner,as I know from his accent.(as指代一个句子,意为“正如”、“就像”)
He has to work on Sunday,which he doesn’t like.(which指代一个句子,没有“正如”、“就像”)
c.当从句的谓语动词是be或相当于联系动词的seem,become等时,可用as或which作主语.但当从句的谓语是行为动词时,只能用which作主语.
例如：
He married her,as (正确) was / seemed natural.
Which （正确）
He saw the girl,as (错误) delighted him.
Which （正确）

你首先要搞清楚限制性定语从句和非限制性定语从句的区别，然后再把as的用法搞清楚，这样才能知道他们的可比性。s……as
AS +adj(原级)+AS
AS +adv(原级)+AS
as soon as 一……就
as soon as possible 尽可能快地
as early as possible 尽可能早的
as carefully...