Which one is correct and why?

  1. I will help you once I understand how it works.


  1. I will help you once I understood how it works.
  • This is a duplicate of ell.stackexchange.com/questions/53438/…, but I think the proper action is to merge the questions, not just closing it. Mar 24 '15 at 15:16
  • @DamkerngT. It's not exactly a duplicate. The same person asked it on English SE, and also here. After he asked it here, the other question was migrated. The other one should be merged with this one as you said. Dec 20 '18 at 14:19

The example sentences are not in the past tense.

Instead, they say that you will do <something> in the future, after you have done <something else>. Presumably, you have not yet done that <something else>. They imply that you might not do that <something else>, and that if you don't do that <something else>, you might not do <something>. So the implied order of events is:

  • Now
  • (When you are learning how it works, which might take forever….)
  • You understand how it works
  • (Maybe some more time)
  • You start helping the other person.
  • (When you are helping the other person.)

Sentence #1 is correct. Sentence #2 is incorrect. The following sentence would be correct:

  1. I will help you once I have understood how it works.

Sentence #3 is more complicated than sentence #1. Sentence #3 adds very little meaning to the sentence, so sentence #1 is better than sentence #3.


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