I recently did a post on these forums where I wrote "Mi lernas la latinan depost septembro".

Now I was not sure whether this needed to be "mi lernas" or "mi lernis". English uses "I've learned" (past tense) while German uses "ich lerne" (present tense - which seems more logical to me because I'm still learning, but maybe it's just because that's what I'm used to from German). So which one is the usual/correct way to say something like this? (Of course I could have used "mi eklernis", but what if I wanted to use "depost"?)

I wouldn't use depost, personally. I never use depost. I would say perhaps "Mi lernas la latinan ekde septembro"

I would use present tense, though. Because you are still learning, right? In English I would say "I have been learning" rather than "I've learned".