be to/must - instructions, must have been - deductions about the past, might/could/should/would have been - criticism about the past, needn't have done  - was not necessary to do (but done), didn't need to/have to - lack of necessity, must - strong advice, should/ought to - less strong advice, could - general ability in the past, was/were able to - ability in a specific situation in the past, should/shouldn't - probability, have to/need to - obligation imposed by sb else, could/would/might - suggestions, recommendations,

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