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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 (now). VS He will come (now).
He must be working. VS He must work.
Can we use 'must' to express future certainty like 'will'?
With a stative verb, "must" expresses necessity; with a non-stative verb, "must" expresses obligation. This is parallel to the two senses of "may", which expresses possibility with a stative, but permission with a non-stative. Sometimes the obligation/permission senses are referred to as "moral" senses, and logicians call this "deontic". Linguists have called the necessity/possibility senses "epistemic", but to a logician "epistemic" concerns knowledge and belief.
I don't see a parallel with "will".
You get the necessity/possibility interpretations with progressives, and also with perfects, but that is really a special case of the rule for statives, since although progressive goes only with a non-stative verb, the progressive itself is stative. A perfect is also stative.
Both "He will be coming (now)" and "He will come (now)" express similar expectations about a future occurrence and seem to me to be virtually interchangeable.
But as Armen Ծիրունյան observes in a comment above, "He must be working" is idiomatically quite different from "He must work." The latter sentence simply states a necessity; but idiomatically, the former sentence is speculative. In a particular context, the speculation might take this form:
He's not here when he said he would be, so he must be working later than he thought he'd have to.
There is no actual necessity implied by a surmise expressed in this form. Merriam-Webster's Eleventh Collegiate Dictionary (2003) defines the relevant sense of must as follows:
4 : be logically inferred or supposed to {it must be time}
In this idiomatic usage, the thing that "must" be true is necessarily true only if the underlying logic or supposition is justified.
