6 、as, which 非限定性定语从句

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 。As 一般放在句首，which 在句中。
As we know, smoking is harmful to one's health.
The sun heats the earth, which is very important to us.
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。