I'm not sure I've met him before.
I might/may have met him before.
We are thinking of going to Rio de Janeiro this summer.
We could/might go to Rio de Janeiro this summer.
I didn't study hard enough, and it is possible I will fail the exam.
I didn't study hard enough, and I may/could fail the exam.
Perhaps she'll apply for that job.
She may/might apply for that job.
I think she's Betty's sister.
She could/may be Betty's sister.
He's probably 70 years old.
He may/might be 70 years old.
There is a possibility that they will not be able to come.
They may/might not be able to come.