he thinks
He thinks he can prove it.
D'abord je vais essayer de comprendre pourquoi il pense ainsi.
First, I’m going to try to understand why he thinks that way.
La seule chose à laquelle il pense est de la rencontrer.
The only thing he can think about is meeting her.
I know how he thinks.
Je me demande à quoi il pense.
I wonder what he's thinking.