定语从句中常考的是which引导的非限制性定语从句，只能用that或只能用which引导的定语从句。

Number1

关系词只能用that的情况

a. 先行词被序数词或形容词最高级所修饰，或本身是序数词、基数词、形容词最高级时，只能用that，而不用which.例如：

He was the first person that passed the exam. 他是第一个通过考试的人。

b.被修饰的先行词为all， any， much， many， everything， anything， none， the one等不定代词时，只能用that，而不用which.例如：

Is there anything that you want to buy in the shop？你在商店里有什么东西要买吗？

c.先行词被the only， the very， the same， the last， little， few 等词修饰时，只能用that，而不用which.例如：

This is the same bike that I lost.这就是我丢的那辆自行车。

d. 先行词里同时含有人或物时，只能用that， 而不用which.例如：

I can remember well the persons and some pictures that I saw in the room.

e.以who或which引导的特殊疑问句，为避免重复，只能用that.例如：

Who is the girl that is crying？

f.主句是there be 结构，修饰主语的定语从句用that，而不用which.例如：

There is a book on the desk that belongs to Tom.

Number2

关系词只能用which，而不用that 的情况

a.先行词为that， those时，用which， 而不用that.例如：

What‘s that which is under the desk？

b.关系代词前有介词时，一般用which，而不用that.例如：

This is the room in which he lives.

c.引导非限制性定语从句，用which， 而不用that.例如：

Tom came back， which made us happy.

Number3

关系副词的用法

a.when指时间，其先行词表示时间，在句中作时间状语。例如：

This was the time when he arrived.

b.where指地点，其先行词表示地点，在句中作地点状语。例如：

This is place where he works.

c.why 指原因，其先行词是原因，起原因状语作用。例如：

Nobody knows the reason why he is often late for school.