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When I see examples of 'must' or 'will' showing certainty, I find that the main verb is either a stative verb or a present participle preceded by 'be'. How different are the two sentences in the following pairs:

He will be coming. VS He will come.
He must be working. VS He must work.
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Wikipedia has reasonably good coverage of modal verbs.

Must is stronger than will: must indicates a requirement, whereas will indicates a prediction of future action without regard to its cause. If someone will do something, we don't know whether it's because they have to, or they want to, or it's just a matter of circumstance.

See also RFC 2119 which outlines the specific meanings of the modal verbs MUST, MAY, SHOULD, and SHALL in requirements documents that cite RFC 2119.

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The two phrases with must are actually quite different.

He must be working:

I am expressing certainty that he is currently working.

He must work:

In general, work is something he has to do.


The two phrases with will are similar, with only a subtle difference.

He will be coming:

I am expressing certainty that he will come, and am considering that action (of him coming) as continuous; that is, I am thinking of it happening over some time period.

This suggests certainty that his transit will occur.

He will come:

I am expressing certainty that he will come, and am considering that action (of him coming) as discrete; that is, it is one complete action in my mind.

This suggests certainty that his arrival will occur.

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The word 'must' in many cases seems to me as lacking of a choice in the matter, like 'have to'. My more used word 'will' allow freedom of choice.

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