So, my translation, "have you seen my new sweater," was not accepted. Was that because of the aspect? Would it have to be perfective?
Truth be told, Ukrainian and English aspects have a complicated correspondence, which is totally not 1-to-1. I think your answer works, too. Basically, when you are randomly referring to the fact of something happening, regardless of the moment in time it happened (eg., whether you saw my sweater or not) it is most natural to use imperfective. English often uses Present Perfect for the same purpose.
A tiny difference is that in Ukrainian you can still specify a rough time frame in such a sentence, which is ungrammatical in English ("Sorry, have you seen my sweater anywhere the day before yesterday, when we were at a party?")
Of course, in that sentence, we would use "did," though I can't remember which English aspect (or mood or whatever) that is supposed to be, since I never took it as a foreign language. I was acceptably conversational in Russian at one point in the distant past, so the distinction is still somewhat familiar to me, but having not put it into practice in decades, my active command of it is essentially gone. Not that I would expect you necessarily to know this, but is the use of aspect pretty similar to that in Russian (or in Czech, which I also took in the distant past)?
I can't see how old this question is, because I'm on the mobile app. However, I used the "Have you seen..." format today, and it was accepted. I think it's possible that the reason for the rejection may have be that they were looking for the word "pullover" instead of "sweater;" I've seen this happen before in numerous languages.
I haven't seen it explained in the lesson, so I'll try asking here: for the past tense, is it a -ила ending if the person associated with the action is female, -ив ending if they're male, and -или if there are multiple people?
Kind of. Except they are «-в», «-ла», «-ли», «-ло» (compare with був, була, було, були) ^_^
The "-и" that you see here belongs to the stem and is the same as in infinitive, as far as I can see. So, if you know the infinitive is фотографувати, the past forms are inevitably «фотографував», «фотографувала», «фотографували» and «фотографувало».