could / (be) able to - describe past ability, should have / shouldn't have - criticise past behaviour , must - express certainty about the present, must have - express certainty about the past, might - talk about a future possibility, could have / may have - talk about possibility in the past, have to / need to - talk about a present obligation/necessity, didn't have to / didn't need to - describe a past lack of obligation or necessity, needn't have + participle - talk about something that was done but it wasn't necessary, didn't have to - talk about something that wasn't done and wasn't necessary,

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