1.先行词是all, many， much ,little, some, no, none，any, everything , something, anything, nothing等不定代词或被不定代词修饰时:
I am going to buy something that I need.
This is all that I know.
This is the best film that I have ever seen.
The first thing that I should do is to get tickets.
They talked about the teachers and schools that they visited.
5. 先行词被the very, the only, the same, the last修饰时，（定高序双特）
This is the only thing that we can do now.
Which is the car that killed her?
Who is the person that is standing at the gate?
He is not the man that he was.他已经不是过去的他了。
Our school is no longer the school that it used to be.
Here is the book that you have been looking for.
There is a seat in the corner that is still not taken.