We use could to show that something is possible in the future, but not certain:
If we don’t hurry we could be late. (i.e., perhaps/maybe we will be late)
Why is it incorrect to say this:
I couldn't be present tomorrow'? (i.e., Perhaps I won't be here tomorrow).
Best Answer
Firstly, it's correct to say I couldn't be present tomorrow, but only if the pragmatic context makes sense for the listener. It will not fly as an example sentence in an English class because these tend to have "out of the blue" background information, and some modal expressions require background information to be felicitous.
When the modal verb is in its preterite form, it expresses remoteness rather than past time. The only different in meaning between I can't be present tomorrow and I couldn't be present tomorrow is that in the first case, the speaker's presence is not negotiable. In the second case, it could be that they are open to incentives, or that they are trying to dissuade the listener from formally asking them to be present.