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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