I have a Japanese friend who is learning English and recently posed me (a native speaker) a question that I am having trouble answering. The problem revolves around two statements:
I was able to get the photos done before I left today
(A contrived example)
He asked you to detain me until he got here, right?
(From an H.G. Wells work)
We started with the former example, where I pointed out that this sentence implies he has already left. My friend countered with the latter example, where the sentence does not make this implication, and he may not have left yet.
I agree with his statement, and believe that the latter is colloquial, albeit not grammatically correct. However what I am having trouble understanding is why the latter is colloquial and not the former. Both of the sentences look grammatically the same to me (past tense followed by a preposition followed by past tense). I believe that no native English speaker would ever say the former example in a scenario where they have not already left, but I could see some people saying the latter in a scenario where he has not left yet (or where his state is unknown).
Is there a concrete reason why the latter is accepted in this scenario where the former is not?