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Which structure in the second paragraph below is grammatical? I provide the first paragraph to contrast with a familiar but opposite structure.

Positive deduction

I didn't get the results I expected. I guess, you must have done something wrong somewhere in the code.

Negative deduction

I asked you to fix the code, but I do not see it working. I saw you playing with the Play station last night. [You must have not done anything / You mustn't have done anything] to fix it.

Source: contrived examples

  • Cat, there is no need to leave the house. :) – Lambie Mar 3 at 18:17
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Technically speaking, both of OP's alternatives are syntactically valid, and in context would mean exactly the same thing.

But idiomatically, native speakers tend to avoid using must in contexts where they mean it must be the case that X did / didn't do Y (as opposed to X must [not] do Y = X is/was required / not permitted to do Y).

The must form was more likely a century or two ago, but it's worth pointing out that even competent native speakers today can be a little uncertain about exactly how this verb works (see Is “must” ever grammatical as a past tense verb? over on ELU). Perhaps partly because of that, we're more likely today to switch the verb to...

You can't have done anything to fix it (or equivalently, ...couldn't have...)

...which has the additional benefit of nullifying the problem of whether or not to use the contracted form (since it would be unusual / stilted to use cannot or can not in this context).


The fact of negation is almost certainly relevant here - most native speakers would agree that in practice, the opposite of It must be true is nearly always It can't be true (not It mustn't be true).

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The second one is correct:

You mustn't have done anything to fix it.

Not sure if there is a general rule, but at least with the construction "must have done", if you want to negate it, the "not" comes after the "must." The "negative" part could come after the "must have" part if you replace either "to do" or "anything" with a word that is basically the opposite, e.g.:

You must have done nothing to fix it.

You must have neglected to fix it.

These both have more or less the same meaning as the original sentence.

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You must not have done anything

is the usual way, in American English at least, to express that thought. In general, the negative indicater comes immediately after the modal.

You must have not worked on the code at all because the results are still ridiculous

is a way to emphasize the failure to do something specific.

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