a.x05as引导非限制性定语从句,只代表主语或主句的一部分所表达的意思(不能指代某个名词或代词),可以放在主句前,后或中间,在从句中作主语或宾语、例如：
As is known to all,he is the best student in our class.(as指代整个句子,作主语,置于句首)
He opposed the idea,as could be expected.(as 指代整个句子,作主语,置于句末)
As we all know,he studies very hard.(as 指代整个句子,作宾语,置于句首)
Taiwan is,as you know,an inseparable part of China.(as指代整个句子,作宾语,置于句中)
3.x05在非限制性定语从句中,which和as引导定语从句时的区别：
a.x05which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)x05is often the case,he is absent.
Which (错误)x05
He saw the girl,x05as (正确)x05He had hoped.
x05Which （正确）x05
b.x05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.x05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,x05as (正确)x05was / seemed natural.
x05Which （正确）x05
d.x05
He saw the girl,x05as (错误)x05delighted him.
x05Which （正确）