They will never forget what you did.
They will never forget what you have done.
Which one is correct?
Learn English – They will never forget what you did / what you have done
past-tense
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Best Answer
The use of simple past implies that the action was completed some time ago and had no consequences. This is odd as, apparently, "they will never forget", so the consequences are lasting.
I don't want to rule it out completely though, because somebody might say this to make it clear that they are talking about one specific event.
Present perfect simple implies that some action was started in the past and it continues to the present, or that the action was completed but the consequences continue. This makes far more sense.