1

I used to have a list that showed the increasing strength of these words, but I seemed to have lost it.

The words/phrases are: must/mustn't, should/shouldn't, have to/don't have to, can/can't, and are allowed to/aren't allowed to.

2

When used in the context of an external rule prohibiting or requiring action, then these three would be ranked as follows (most imperative to least):

  1. must
  2. have to
  3. should

should is also a boarderline case here. should implies that there would be some negative consequence if not heeded, but few would argue that there's an external force imposing a rule as with must and have to. Also note that both must and have to can be used in other contexts which do not imply any rule, but rather a value judgement or self-imposed rule.

can & allowed are based more on permission being granted or absence of rules prohibiting rather than on external requirements. The differences between them are more in formality rather than differences in permission, with the former being less formal.

  • By putting 'must' first did you mean that it's the strongest? – Istable Sep 1 '11 at 10:32
  • Yes, sorry. I've tried to make that clearer now. – user12549 Sep 1 '11 at 10:37
  • I agree. "Must" and "have to" are almost the same and the most imperative. "Should" would be next. "Can" and "allowed" would have no imperative connotation at all – Kevin Sep 1 '11 at 15:38

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