1、先行词前面有形容词最高级时，例如：
This is the best film that I have seen.
2、先行词被序数词修饰时，例如：
The first place that they visited was the Big Ben.（ 3、先行词是以下词：anything/everything/nothing/all等词时，例如：
All that can be done has been done.
1.先行词前有序数词修饰时2.先行词前有最高级修饰时3.先行词是不定代词时4.先行词既有人也有物时5.先行词被不定代词修饰时6.先行词有only修饰时
