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.