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which one is correct and why ?

1- I will help you once I understand how it works or 2- I will help you once I understood how it works

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    This question is better suited to our sister site, English Language Learners. At the moment we can not automatically migrate it there, but we can close it here so you can go re-ask it on that site.
    – Dan Bron
    Mar 24, 2015 at 0:02
  • Sorry for posting it here I will ask there
    – Shahab
    Mar 24, 2015 at 0:32
  • Yes, "I will help you once I understand how it works". Mar 24, 2015 at 10:23
  • This is a duplicate of ell.stackexchange.com/questions/53419/…, but I think the proper action is to merge the questions, not just closing it. Mar 24, 2015 at 15:16

1 Answer 1

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Where is the past tense? You are using a future tense sentence, the understanding (and help) will come in the future. You first need a sentence that has past-tense meaning before asking about the proper usage. However, this is correct:

I will help you once I understand how it works

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