Certainty, Probability, and Possibility (Modal Verbs)

The modal verbs are must, can't, should, ought to, may, might, and could.

The modals are used to show that we believe something is certain, probable or possible:

Certainty:

We use the modal must to show we are sure something to be true and we have reasons for our belief:

It’s getting dark. It must be quite late.
You haven’t eaten all day. You must be hungry.

We use the negative can’t or cannot to show that something is impossible:

That can’t be true.
You cannot be serious.

Probability:

We use the modal should/ought to to suggest that something is true or will be true in the future, and to show you have reasons for your suggestion:

Ask Miranda. She should know.
It's nearly six o'clock. They ought to arrive soon.

Possibility:

We use the modals could, might and may to show that something is possible in the future, but not certain:

They might come later. (= Perhaps/Maybe they will come later.)
They may come by car. (= Perhaps/Maybe they will come by car.)
If we don’t hurry we could be late. (= Perhaps/Maybe we will be late)

SOURCE: https://learnenglish.britishcouncil.org/en/english-grammar/verbs/modal-verbs/certain-probable-or-possible