The reason (that) she did it is still a mystery. 或 The reason why she did it is still a mystery.
We have every reason to refuse.
We have reason to believe that he was lying.
The reason for her absence was that she was ill.
Why won't you listen to reason?
He is good at teaching his students to reason.
I reasoned that he did not come because his old car had broken down again.
I reasoned him out of selling his house.