Which one is correct and why?
- I will help you once I understand how it works.
- I will help you once I understood how it works.
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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:
Sentence #1 is correct. Sentence #2 is incorrect. The following sentence would be correct:
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.