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I know the difference between

1) I needn’t have done it (but I did it) – I didn’t need to do it (and I didn't do it.)

But do these make sense? I have come across them being uttered or written by native speakers

2) I need to have done it. – I needed to do it. (What's the semantic difference?)

3) Need you have done it? – Did you need to do it? (What's the semantic difference?)

2

In your 2) I need to have done it is a present-tense use of need and is only meaningful with a future reference, such as I need to have done it by Friday, because I'm leaving town on Saturday. I needed to do it is a past-tense use of need and refers to a past necessity, such as *I needed to do it right away, because I was to meet the client that afternoon.

The two sentences in your 3) mean the same thing. Note that the first sentence employs need as a modal, taking the bare infinitive (without to). This is an old use which in effect treats the present form as immutable, like must: the have done construction is not a true perfect but a perfect employed as a modal past marker. Need have VERBed has been steadily declining since about 1900, and is nearly defunct now.

  • Can the 3 mean that I am pretty sure that my addressee didn't need to do it and I am king of rebuking him? – user1425 Nov 23 '13 at 15:15
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    @user1425 It may mean that, and usually does; but it may be a simple question. That will depend on context. – StoneyB on hiatus Nov 23 '13 at 15:29
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In your #2, it likely would be read as past perfect "have done it". Since, as StoneyB pointed out, this is only meaningful with a future reference, the example as you gave it is nonsensical. Either you have done it or you haven't; you cannot change the past, so you can't "need" to change it. The best you can do is to wish you had, by saying "I should have done it."

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