word usage - Shift to "must" for negation of "have to"?


According to englishpage.com, if have to or must expresses certainty, the negative form uses must not. Example:



That has to be Jerry. They said he was tall with bright red hair.
=> That must not be Jerry. They said he has blond hair, not red hair.



I learned that must not expresses prohibition (and this case is also listed there) and I've never encountered an exception so far. So, it sounded funny to me and I asked a native English speaker. She's of the opinion that this usage of must not is not right, but she's only sure about British English.



  • Is this usage of must not just wrong?

  • Or is it correct for a certain variation of English? American English, for instance?

  • Or was it correct in former times and nowadays considered archaic?


EDIT: Many, if not all, answers state that the proper use is can't. This, however, is not my question. Furthermore, some answers elaborated on the proper meaning of must not in the sense of "not allowed to do". This isn't my question either. I must be aware (no pun intended) of the proper meaning of must not, otherwise I wouldn't have noticed this 'error'.



I don't know nothing about the author of that page but, in dubio pro reo, I assume that they know what they talk about.



Answer



While I agree that "that can't be Jerry" is idiomatic, there are times when we do use "must not".


Let's say you're watching a murder mystery. You get clue after clue, and eventually come to suspect that a particular character is the villain, let's say it's the pharmacist. But then a new clue surfaces, which seems to point suspicion elsewhere. You can say:


So, it must not be the pharmacist.


The phrase above is spoken tentatively, with a questioning intonation, because you are only coming to believe something but are not quite certain.


When you do become certain that the pharmacist is not the murderer (e.g. he's having an affair with the mayor, and they were in Las Vegas at the time, and the two of them are clearly visible on the security TV tapes of the casino at the time of the murder), then you can say:


So, it can't be the pharmacist!


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