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Q) It was very kind of you to do the washing up, but you -- it

Should it be "did not have to do" or "might not have done" or "must not have done"?

They all seem to convey same meaning in general to me. Which is correct grammatically?

  • This makes no sense at all. Could you try to revise this and then we might be able to help: Q) It was very kind of you to do the washing up. but you -- it did not have to do or might not have done or must not have done – Great Crosby Apr 7 '16 at 18:50
  • Just to confuse matters... the more common idiomatic forms for this would be "you needn't have done it" - there was no need for you to do it, and "you shouldn't have done it" - (loosely speaking) you have committed a minor breach of protocol by doing it, but thanks anyway. – JavaLatte Apr 7 '16 at 20:21
  • I suspect that you speak German, or another language where the equivalent of "müss nicht" means "doesn't have to". This is a false friend in English: "must not" means "darf nicht". – Colin Fine Apr 7 '16 at 20:37
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It was very kind of you to do the washing up, but you did not have to do it.

converts to

Your action was not necessary.

To have to is to be obliged to1.

It was very kind of you to do the washing up, but you might not have done it.

converts to

It is possible that you didn't do it.

Might is "used in auxiliary function to express permission, liberty, probability, possibility in the past"2

It was very kind of you to do the washing up, but you must not have done it.

converts to

I have come to the conclusion that you didn't do it.

Must is "used to indicate logical probability or presumptive certainty"3


1http://www.dictionary.com/browse/have-to
2http://www.merriam-webster.com/dictionary/might
3http://www.thefreedictionary.com/must

  • 3
    There's also a more regular but quite old-fashioned negation which meets OP's need: You need not have done it. – StoneyB on hiatus Apr 7 '16 at 20:44

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