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Please help me. Which of these is correct?

  1. Let me know once you finish

  2. Let me know when you finish

  3. Let me know when you have finished.

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    Hi, Happy, and welcome to EL&U. Proofreading is Off Topic here. Please feel free take the site tour and visit the help center for guidance on how to use this site. :) – anongoodnurse May 30 '14 at 7:32
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    This is clearly not a general proofreading question, but a very specific question of which verb tense to use, and thus it should not be closed on those grounds. "Proofreading questions are off-topic unless a specific source of concern is identified". By listing several possible choices of verb tense, the OP has clearly (implicitly) identified the source of concern. – Peter Shor May 30 '14 at 12:30
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    Note: there has been a remarkable change in usage of these phrases historically. See Ngrams. Traditionally, only "when you have finished" is correct, but in modern grammar, I would say all these choices are. – Peter Shor May 30 '14 at 12:36
  • @PeterShor +1 for the Ngram. Supplement a reasoned answer with that and post it. – njboot May 30 '14 at 18:38
  • @njboot: I don't have a reasoned answer. It feels to me that "when you finish/have finished" and "when you arrive/have arrived" should behave in exactly the same way. But the usage pattern is completely different. I'm baffled. – Peter Shor May 30 '14 at 18:46
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Here is the Google Ngram, courtesy of @Peter Shor (slightly modified): enter image description here

Based on this Ngram, I'd say that "when" is clearly the preferable adverb in this context. While I don't believe the first sentence is grammatically incorrect, it's awkward.

Sentences two and three are idiomatically correct - that being, widely accepted and natural sounding.

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It of course also depends on whether it's American English or British English:

British: British English ngram American: American English ngram

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All three are acceptable; at least without more knowledge of the context.

"when you have finished" possibly implies that the action is more final and firmly in the past than "when you finish" or "once you finish", e.g. "when you have finished your homework, you may play outside" (it must all have been completed properly) vs "when you finish school, I'll collect you" (I'll be waiting for you when the bell rings).

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