由as,which 引导的非限定性定语从句,as和which可代整个主句,相当于and this或and that.
区别：
1、as可位于主句的前、中（做插入成分）、后,译为“正如”.
which只能在主句中、后,一般不译.
As we all know ,he never smokes .
正如我们所知,他从不吸烟.
2、如果从句是否定句,则只能用“which”引导.
3、非限定性定语从句中出现expect ,think ,suppose 等表示猜测、想象、预料等时,只能用as
She succeeded in her doing the research work ,as we expected .
4、固定搭配：such/the same+as
典型例题
1）Alice received an invitation from her boss,___came as a surprise.
A.it B.that C.which D.he
答案C.此为非限定性从句,不能用 that修饰,而用which.,it 和he 都使后句成为句子,两个独立的句子不能单以逗号连接.况且选he句意不通.
2）The weather turned out to be very good,___ was more than we could expect.
A.what B.which C.that D.it
答案B.which可代替句子,用于非限定性定语从句,而what不可.That 不能用于非限定性定语从句,it不为连词,使由逗号连接的两个句子并在一起在英语语法上行不通.
3）It rained hard yesterday,____ prevented me from going to the park..
A.that B.which C.as D.it
答案B.
as 和which在引导非限制性定语从句时,这两个关系代词都指主句所表达的整个意思,且在定语从句中都可以作主语和宾语.但不同之处主要有两点：
(1） as 引导的定语从句可置于句首,而which不可.
(2） as 代表前面的整个主句并在从句中作主语时,从句中的谓语必须是系动词；若为行为动词,则从句中的关系代词只能用which..
在本题中,prevent由于是行为动词,所以正确选项应为B.
As 的用法
例1.the same… as；such…as 中的as 是一种固定结构,和……一样…….
I have got into the same trouble as he (has).
例2.as可引导非限制性从句,常带有'正如'.
As we know,smoking is harmful to one's health.
As is know,smoking is harmful to one's health.
As是关系代词.例1中的as作know的宾语；例2中,它充当从句的主语,谓语动词know要用被动式.
这是我经过资料整合的结果,