"Why are you always late?"
Translation:Почему ты всегда опаздываешь?
I tried that, and for some reason it was marked worng. Im no russian expert, can someone please explain
It's not wrong, but it's not very common to leave out pronouns in Russian. That doesn't mean the sentence is grammatically wrong, of course. "Почему всегда опаздываешь?" actually sounds kind of aggressive to me, but that's just my opinion.)
What about Почему всегда ты опаздываеш? Got it wrong, but wouldn't it work as well?
Is it possible to say Почему ты всегда опоздал/опоздала? Or is that ungrammatical/weird sounding?
No, it is not correct. You need to use imperfective verbs to express habitual actions.