初中英语定语从句之非限制性定语从句

　　as, which 非限定性定语从句

　　  
由as, which 引导的初中英语定语从句非限定性定语从句，as和which可代整个主句，相当于and this或and that。As个别放在句首，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. 此为初中英语定语从句中的非限定性从句，不能用 that修饰，而用which.，it 跟he 都使后句成为句子，两个独破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，使由逗号衔接的两个句子并在一起在英语语法上行不通。

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3）It rained hard yesterday, ____ prevented me from going to the park..
A. that　B. which　　C. as　 D. it
谜底B.

as 和which在勾引非制约性定语从句时，这两个关系代词都指主句所表白的全体意思，且在定语从句中都可能作主语和宾语。但不同之处主要有两点：
(1） as 领导的定语从句可置于句首，而which不可。
(2） as 代表前面的全部主句并在从句中作主语时，从句中的谓语必须是系动词；若为行动动词，则从句中的关联代词只能用which.。
在本题中，prevent由于是举动动词，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, 跟……一样……。
I have got into the same trouble as he (has).

例2. as可引导初中英语定语从句的非限度性从句，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。

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