(3)先行词前由形容词的最高级或序数词修饰时，只能用that。
例如：This is the most interesting book that I have ever read.
(4)先行词同时包括人和物时，只能用他和她，
例如：Lu Xun and his works that we are familiar with are considered great.
6. 只能用which不能用that的情形
(1)当非限制性定语从句中的先行词指物时，只能用which
例如：The pencil-case, which I bought last week, is missing.
(2)关系代词前面有介词时，只能用which
例如：The tree under which we used to take a rest has been cut down.
(3)先行词本身就是that时
例如：I don’t like that which he did.

关系副词的用法 
1. when的用法：它的先行词通常是time, day, morning等。有时也可和一些介词一起引导定语从句。
例如：Do you remember the time when she went abroad?
2. where的用法：它的先行词通常有place, spot, room等。
例如：I like places where the weather is always warm.
3. why的用法：它的先行词只有reason。Why时常也可以省略。
例如：Tell me the reason why she was late.