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Both forms express an obligation not to do something (although one phrases it by declaring that permission is missing). I figure that must not sounds a little more urging but what I am interested in is this:

Are both phrases semantically interchangeable or are there situations in which only one will work? If so, why?

  • I find must not sounds LESS urging and more as a suggestion than the disallow – mplungjan Dec 4 '13 at 9:37
  • Necessary modals (must, shall, should), and their paraphrases (have to, be allowed to, ought to) have a deontic sense implying an obligation. They can be distinguished by shades of obligation (light to heavy), the imposer of the obligation (an Authority, general carefulness, or the laws of Nature), and the feelings of the speaker about the obligation and the necessity to follow it. – John Lawler Dec 4 '13 at 16:24
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The phrases do not mean the same thing, and while they may often be used interchangeably, this is not always the case.

You must not put your fingers in the spinning gears!

For adults, this is an admonition based on objective reality. It really means

You must not put your fingers in the spinning gears [or you will get injured]!

But in many situations, there is no authority that controls this activity. If we are talking about your machine in your home, it really doesn't make sense to say

You are not allowed to put your fingers in the spinning gears!

Not allowed by whom?

For children, we often impose an authoritarian limitation because they don't yet understand the practical limitation, and so the terms are often substituted. Also, for adults, practical and authority restrictions often coincide.

You must not run a red light.

You are not allowed to run a red light.

Sometimes must not is used to indicate lack of permission by an authority, even where there is no real practical risk

You must not fail to salute passing flag.

In short, for the vast majority of circumstance where not allowed to works, must not will also work. For some circumstances where must not works (where there is no rule, but real practical downsides), not allowed to does not.

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Not allowed to indicates that some external agency is imposing whatever the prohibition happens to be. Must not is much less clear in this respect.

  • Ok, that was kind of obvious I guess, but if there is such an agency, would you then consider both phrasings the same or is there still a difference (I am asking because we were talking about 'nicht brauchen' vs 'nicht müssen' over at german.stackexchange) – Emanuel Dec 4 '13 at 9:48
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    If there was such an agency, my guess is that most speakers would use not allowed to. I certainly don't think the two are interchangeable. The choice depends on the context, and on the emphasis the speaker wants to give the prohibition. – Barrie England Dec 4 '13 at 9:59
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'Must' implies that the speaker feels strongly about the rule himself and is urging that it be followed; you use it when you strongly want the listener to do something. Whereas 'not allowed ' simply means there is a known rule against it. One could say 'we are not allowed to go out at night, but let's do it anyway.' It's more like 'should' .

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