3 、非限定性定语从句只能用which 不用that

　　非限制性定语从句中只能用which，可以指代前面的一个词，也可以引导从句修饰前面的整个主句，代替主句所表示的整体概念或部分概念。在这种从句中，which可以作主语，也可以作宾语或表语，多数情况下意思是与and this 相似。例如：

　　He succeeded in the competition, which made his parents very happy.

　　他比赛获胜，令他父母十分欣慰。

　　4、 As在定语从句中的用法

　　1)引导限制性定语从句和非限制性定语从句

　　(1)as多与such 或the same连用，可以代替先行词是人或物的名词。

　　such… as; same…as

　　It wasn’t such a good dinner as she had promised us

　　(2)as 也可单独使用，引导非限制性定语从句，作用相当于which。例如：

　　The elephant’s nose is like a snake, as anybody can see.

　　2)As引导的非限制性定语从句的位置

　　as引导的非限制性定语从句位置较灵活，可以位于主句前面.中间或后面，一般用逗号与主句隔开，但which所引导的非限制性定语从句只能放在主句之后

　　As we all know, Taiwan is a part of China.

　　Water consists of hydrogen and oxygen, which (as ) is known to all.

　　5 、做介词宾语时，如介词位于关系代词前一般用which 或whom, 如介词位于句末，也可使用that,且可省略。

　　The earth on which we are now living came into being long, long ago.

　　The earth (which/that) we are now living on came into being long, long ago.

　　6 、误区

　　That was the time that/which I still remember today.

　　That was the time when(at which) he came.

　　That is the factory that/which we are going to visit.

　　That is the factory where(in which) my brother works.

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