1.先行詞是all, many， much ,little, some, no, none，any, everything , something, anything, nothing等不定代詞或被不定代詞修飾時:
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. 先行詞被the very, the only, the same, the last修飾時，(定高序雙特)
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.他已經不是過去的他了。
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.