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Is it possible to use 'must' or 'should' to express certainty in the future? Is the sentence 'He must/should be coming tomorrow.' correct?

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Should is never certain; it indicates a correct or expected action to be taken.

Must indicates a requirement.

The only problem with your sentence is that by adding be to the modal verb, must be and should be weaken the certainty, and indicate that the speaker is guessing or making a deduction. I'm not sure when this connotation came into being; the literal meaning of must be and should be is strong, but sometime we started to use must be and should be as a weaker expression of certainty.

(This is covered in the Wikipedia article on modal verbs; epistemic modality indicates a degree of belief, whereas deontic modality indicates a degree of requirement.)

The strongest modal verb in English is shall; it's rather formal though.

If you just want to express certainty but not cause, it's fine to say He will be coming tomorrow.

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  • Epistemic should' can express a degree of certainty. The certainty is, for example, greater than that expressed by may/might and not as great as that expressed by will. I don't know what you men by 'The strongest modal verb in English is shall'. Strongest in what way? – tunny Nov 9 '14 at 17:28
  • The "requirement" aspect of must is expressed by "He must come tomorrow". There seems to be little or no difference in meaning between will come and will be coming as replacements there. But as you say - if we switch to must be coming it suddenly becomes a lot less of a certainty/requirement. It's just a slightly stronger way of saying "[Given what I now know,] The only outcome/explanation I can think of is that he will come tomorrow". – FumbleFingers Nov 9 '14 at 17:32

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