grammar - Proper way of using past tense

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.

Answer

The example sentences are not in the past tense.

Instead, they say that you will do in the future, after you have done . Presumably, you have not yet done that . They imply that you might not do that , and that if you don't do that , you might not do . 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:

3. 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.