引導定語從句的關系代詞who,which有時可與that互換使用。但在下面情況，一般只用that。

一、當先行詞為不定代詞anything, everything, nothing, any, little, one, few, much, all, none等時。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、當先行詞是形容詞最高級或被形容詞最高級修飾時。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、當先行詞是序數詞或其被序數詞修飾時。如：? a.It is the third one that I’ve bought. b.This is the first place that I’ve ever visited.

四、當先行詞被the very, the only等詞修飾時。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、當先行詞包含了人和事物兩方面的含義時。如：? They talked of things and persons that they remembered in the school.

六、當先行詞被same修飾，且指“同一物品”時。如：? She is wearing the same dress that she wore yesterday.

七、關系代詞在定語從句中作表語時。如：? a.This is no longer the place that it used to be. b.He is not the man that he was.

八、當主句是以who,which開頭的特殊疑問句或先行詞是who時。如：? a.Which is the book that he bought yesterday? b.Who that you