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.
He is not the man that he used to be.
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.
如：They set up a state for their own , where they would be free to keep Negroes as slaves.
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 is know, smoking is harmful to one's health.
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