Future Possibility and Probability Modals
Will have to, have to and must convey the idea that an action is going to be necessary in the near future. The negative forms are will not and do not have to. For example:
The movie starts at 9:00 and it’s already 8:15. We will have to leave soon, or we will miss the beginning.
There’s no hurry. The movie doesn’t start until 9:00, and it’s only 6:45. We do not have to leave for a while yet.
Should and ought to convey the idea that something is expected in the future. For example:
As far as I know, the ferry is running on schedule. We should arrive in Vancouver in about an hour.
Let’s stop by their house on the way home. They ought to be home by the time we get there.
Could, may or might convey the idea of future possibility. Of these, may expresses a stronger degree of certainty that an event will occur. For example:
The temperature is dropping. It could snow during the night.
To convey the idea that a future event will possibly not take place, use may not or might not. Do not use could not.
Correct: The temperature is rising. It may not snow tonight after all.
Incorrect: The temperature is rising. It could not snow tonight after all.