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If it was not necessary for someone to do something on a particular occasion, you say that they didn't need/have to do it, but in a reporting clause you can use needn't do it, as in They knew they needn't bother about me.

If someone has done something and you want to say that it was not necessary, you can say that they needn't have done it.

https://www.wordreference.com/EnglishUsage/need

  1. Can didn't need/have to do it also be used if someone has already done it but it was not necessary?

  2. In They knew they needn't bother about me, is needn't really referring to a past situation, so that we can add ...but now they do because I am currently in trouble.

1 Answer 1

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If someone has already done it, but it was not necessary, you'd convey that with negated Present Perfect. Which as usual, implies a (potentially negated) past action with current relevance...

They knew they needn't have bothered about me

But note that the abbreviated form needn't has been steadily losing traction over recent decades, giving way to the "do- support" forms don't/didn't need to1...

enter image description here

I'm actually comparing the abbreviated forms you needn't and you don't need to above, but NGrams chooses to expand the text shown on the chart (it is doing the intended text matching).

I won't show the corpus-specific charts here, but you can do that yourself if you follow my NGram link above. And I'll just mention that this usage shift is in fact more pronounced with the British English corpus. Which I find surprising, since it's always been my impression that American English favours more "function words" and "auxiliary verbs" than BrE. That may have been true 50-100 years ago (in the kind of texts I grew up with), but it seems to no longer be the case.


1 Note that in OP's specific example, my preferred phrasing would be...

They knew they didn't need to bother about me
...as opposed to...
They knew they didn't need to have bothered about me
...because that full-blown Present Perfect alternative is a bit too "wordy".

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