Could is used to express possibility.
Mary could read without glasses when she was younger.
Mary could not put the book down.
Could is used to make a request. It is always in the question form.
Mary speaks softly. So I ask her, “Could you repeat that please?”
Could is used to make a suggestion. It is only used in the affirmative.
What do you suggest Mary reads now?
She could read “Through the Looking Glass“?
Might is used to express a possibility in the present or future.
It might rain later.
You can also use: can, could, but, may and might are more common for possibiities.
Mary might not finish reading the book later.
Might is used also as a possibility in the past. However, it is followed by: have done.
Mary might have fallen asleep before she reached the end of the chapter.
Mary might have visited you, if you were home.
Mary could have visited you, if you were home. the train – no negative form.