may

[azz]

a. He might come into our room.
b. He could come into our room.

Could the above sentences be used to mean:
He had the permission to come into our room.

c.He mightn't come into our room.
d. He couldn't come into our room.

Could the above sentences be used to mean:
He didn't have the permission to come into our room.

[Tdol]

'Might' is not used much for permission. It would be more common in both examples to use 'may' instead. I would understand both examples with 'might' to refer to possibility not authorisation.

[azz]

But one can't use "may" when one is talking about the past. I think "could" is OK though.