I learned that the past form of 'must'(obligation) is 'had to', but there is no past form of 'must not'.

So, "He says that I must do it." can be changed to "He said that I had to do it." Then, "He says that I must not do it." should be changed to "He said that I must not do it"?

"didn't have to" means "didn't need to", and "must not have pp" means the past form when it indicates being sure about something in the past (can't have pp/couldn't have pp is more commonly used, though). So, I don't think neither can be the answer. Then just "must not" even in the past tense?

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    Yes, we would use must not (or mustn't) in that case.
    – Colin Fine
    Commented May 8, 2023 at 15:42
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    "Must" is a "defective" verb - the Past and Present tenses are the same. Bizarrely, though, must was originally the past tense of mote - which we still encounter sometimes with Freemasons and such saying So mote it be ( = It must be so, Let it be so). See this ELU question about the usage. Commented May 8, 2023 at 16:10
  • We can't use must to mean didn't have to, if that is what you are asking. Commented May 8, 2023 at 16:55

1 Answer 1


Your question confused me a bit, but I think that you're asking how to form the past tense version of "He says that I must not do it."

Yes, you can write "He said that I must not do it." As FF says in a comment above, "must" was originally a past tense (preterite) form, and it can still be used for past action.

It's sometimes possible to write "had to not". In this case, I don't think that doing so ("he said that I had to not do it") would work well, but it depends on context.

Of course, you can also simply use different words. For example: "He prohibited me from doing it."

  • Good point about how to use have to (=must) as a negated past tense. The "headline" point is that He said I didn't have to go doesn't mean the same thing as He said that I mustn't go - but we can convey the second sense there with He said that I had to not go (or ...that I had not to go ?? - which I don't mind, but opinions may differ on that one). Commented May 8, 2023 at 16:58

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