Here we discuss structureThe way in which the parts of a sentence, clause or expression are arranged. E.g.: make somebody + infinitive in "The teacher made me rewrite the composition."s that show how certain the speaker is whether something happens or will happen. Between the two extremes (the first and the last sentence below) the order of the structures (might, may, could etc.) expresses a growing amount of certainty.

He can't be at home. (= I'm sure he is not at home.)
He might be at home.
He may be at home.
He could be at home.
He is likely to be at home. It is likely that he is at home.
He must be at home. (= I'm sure he is at home.)

Should can also express uncertainty if used in questions, which can be interpreted as asking for advice.

'Should I stay or should I go?'

Note that for future certainty we use be bound to.

He is bound to be at home at 5. (= I'm sure he will be at home at 5.)

Modals expressing past certainty / uncertainty
Certainty / uncertainty with modals Criticism with modals

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