1、当先行词是不定代词all，little, few, much, anything , everything, none, nothing, no one, some等时。
He did everything that he could to help us.
2、先行词被all, every, no , some, any , little, much, the only, the very, the right, last , few , just 等修饰时。
The only thing that we can do is to give you some advice.
3、先行词被序数词或形容词最高级修饰时，或先行词本身就是序数词或形容词最高级时。
This is the most interesting film that I’ve ever seen.
