18.6 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。

18.7 先行词和关系词二合一

　1)Whoever spits in public will be punished here.

　　 (Whoever 可以用 anyone who 代替)

　2)The parents will use what they have to send their son to technical school.

　　(what 可以用all that代替)

18.8 what/whatever;that/what; who/whoever

1)what = the thing which；whatever = anything

　　　What you want has been sent here.

　　　Whatever you want makes no difference to me.

2) who= the person that　 whoever= anyone who

　　 （错）Who breaks the law will be punished.

　　 （错）Whoever robbed the bank is not clear.

　　 （对）Whoever breaks the law will be punished.

　　 （对）Who robbed the bank is not clear.

3) that 和 what　

　　当that引导定语从句时 ，通常用作关系代词，而引导名词性从句时，是个不充当任何成分的连接词。宾语从句和表语从句中的that常可省略。What只能引导名词性从　句， 用作连接代词，作从句的具体成分，且不能省略。

　　I think (that) you will like the stamps.

　　What we need is more practice.