He must be rich ( = I'm fairly sure that he is rich)
Modal verb 'must' can indicate strong certainty.
What about 'have to' ?? Does 'have to' have the same meaning as 'must' ?
Some people say that it is grammatically incorrect, but other people say that it is possible
He has to be rich (?)
S. t1%3B%2C(has%20to%20be%20rich%3Aeng_us_2012)%3B%2Cc0 CJ
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'has to' has been used as a substitute for 'must' in that context for more than a hundred years both in Great Britain and in the U.S.
Generally speaking, "must" and "have to" are interchangeable, but don't rely on a fixed rule. Must - is a higher order word than " have to ". In a negative connotation, if I say" you mustn't listen to him", I am telling you without hesitation, vs " you don't have to listen to him" which is a softer statement.