c)先行词为序数词(the last)、数词、形容词最高级时，只用that。

The first English book that I read was "The Prince and the Pauper" by Mark Twin.

d)先行词既有人，又有物时。

He talked about the teachers and schools that he visited.

e)当主句是以who 或which 开始的特殊疑问句时，用that 以避免重复。

Who is the person that is standing at the gate.

f)关系代词在从句中做表语

He is not the man that he used to be.

2 、关系副词引导的定语从句

关系副词可代替的先行词是时间、地点、方式或理由的名词，在从句中作状语。

关系副词when, where, why, how的含义相当于"介词+ which"结构，因此常常和"介词+ which"结构交替使用，例如：

There are occasions when (on which) one must yield.

Beijing is the place where (in which) I was born.

Is this the reason why (for which) he refused our offer?

I'm surprised the way how (by which) he works out the problem.

注意：

①在非限制性定语从句中，"介词+ which"结构不能代替关系副词。

如：They set up a state for their own , where they would be free to keep Negroes as slaves.

②含有介词短语的动词一般不能拆开，介词仍放在动词后面。

Is this the book which (that) she was looking for?

3、名词/数词/代词 /形容词最高级 + 介词 + 关系代词引导定语从句

She has written a book , the name of which I have forgotten.

There are fifty-five students in our class , all of whom are working hard.

There are five continents in the world , the largest of which is Asia.

4、as, which 引导非限定性定语从句的差别

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.

as可引导非限制性从句，常带有“正如”的意思。

As is know, smoking is harmful to one's health.

用法区别：

(1) as 引导的定语从句可置于句首，而which不可。

As we all know, he never smokes.

(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。

(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。

She succeeded in her doing the research work , as we expected.

(4)As 的用法 the same… as; such…as 中的as 是一种固定结构, 和……一样……。

I should like to use the same tool as is used here.

We should have such a dictionary as he is using.

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