引导定语从句的关系代词who,which有时可与that互换使用。但在下面情况，一般只用that。

一、当先行词为不定代词anything, everything, nothing, any, little, one, few, much, all, none等时。如：? a.There isn’t much that I can do. b.He is one of the few that can work it out in five minutes.

二、当先行词是形容词最高级或被形容词最高级修饰时。如：? a.The book is the best that I have read. b.This is the biggest laboratory that we have ever built in our university.

三、当先行词是序数词或其被序数词修饰时。如：? a.It is the third one that I’ve bought. b.This is the first place that I’ve ever visited.

四、当先行词被the very, the only等词修饰时。如：? a.Is this the very museum that you visited the other day? b.This is the only book that I need at present.

五、当先行词包含了人和事物两方面的含义时。如：? They talked of things and persons that they remembered in the school.

六、当先行词被same修饰，且指“同一物品”时。如：? She is wearing the same dress that she wore yesterday.

七、关系代词在定语从句中作表语时。如：? a.This is no longer the place that it used to be. b.He is not the man that he was.

八、当主句是以who,which开头的特殊疑问句或先行词是who时。如：? a.Which is the book that he bought yesterday? b.Who that you