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he may not come to the party.- it is unlikely or he is not allowed
he might not come to the party.- there is a chance he won't come
he could not come to the party.- he was unable to come or he is un likely to come
he could not come to the party.- he is un likely to come
For the life of me, Tdol, I can't think of any situation where this would hold, but admittedly, I'm awfully tired. Could you give me an example please?