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There are only these two possibilities A and B. It was proved that it is not A. So it ... be B.

In the above sorts of sentences, what should be used for the blank: "must", "should" or "have to"?

I found out in my research that "must" and "have to" is for more strong and inevitable obligations such as rules. So, it must be either "must" or "have to"! But I anyway like to know your views about the word choice in my sentence.

  • It must be B. As a modal auxiliary Must is commonly used to express a logical possibility. – mahmud koya Aug 11 '17 at 15:10
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    All modals have two kinds of meaning. One, called Deontic, has to do with social rules, obligations, and permissions: She must be home by midnight or she'll be grounded. The other kind, called Epistemic, has to do with logical or probabilistic statements, not obligations: She must be home by now; it's past 2400. – John Lawler Aug 11 '17 at 15:59
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I have read on a couple of different sites (https://learnenglish.britishcouncil.org/en/quick-grammar/modals-1 and https://www.gymglish.com/en/english-grammar/must-vs-have-to that has to would be the correct term to use in this example.

According to these resources, must is used when the obligation is coming from the speaker, and have to is used when the source of the obligation is someone or something other than the speaker (i.e., another person or a law/rule).

In the example you have used, the obligation is a result of a logical rule, and this is why the sentence should read, "So it has to be B."

However, this answer is based on the information given on a couple of sites that happened to rank high on a google search for "must vs. have to," and so this could be wrong; hopefully, someone will let us know if so.

  • See @JohnLawler's comment on the question.There are two meanings/uses of must, and one of them does apply here, just as much as has to applies here. In this context, must means that it is necessarily so. – Drew Aug 11 '17 at 18:59
  • I'd agree that there are slightly different distributions for deontic 'must' and 'have to'. 'Must' is less hedged. You wouldn't hear 'You have to obey!' from a Dalek. // But this example uses epistemic 'must' and 'have to'. I'd say they're closely synonymous here. – Edwin Ashworth Aug 11 '17 at 19:13

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