What is the degree of strength of the verbs listed below dealing with rules/advice?
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):
- must
- have to
- 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.