While in present tense you can't see the gender of the subject, in past tense and future compound - you can. So here it will be:
He -> (On) jadł.
She -> (Ona) jadła.
It -> (Ono) jadło.
Similarly, if you were speaking directly to someone, it would be "(Ty) jadłeś" when you speak to a man, and "(Ty) jadłaś" when you speak to a woman. That applies to every person, including "I".
I believe that the majority of Polish learners of English would translate it to "supper". I would.
But the 'meals of the day' distinction has proved to be one of the most problematic things in this course. It's a cultural thing between different parts of the English-speaking world. You have (simplifying) the 'American way', which translates śniadanie/obiad/kolacja to breakfast/lunch/dinner. And then you have the 'British way', which translates those to breakfast/dinner/supper.
We decided to always use the American version as the default (while accepting the British one, of course) as Duolingo is an American company. And it turned out to be a good decision, because the amount of complaining/doubts/questions has decreased significantly. But you can always translate "kolacja" to "supper" if that's a more natural answer to you.
Thanks for this explanation. Living in Poland I have noticed everyone has a mammoth lunch whether they are at home or work..the big one....which is different to the English speaking world. Lunch is a small affair. So it makes sense that in Poland the distinction between big obiad small kolacja is different. Your digestive system thanks you for it, as it causes an upset stomach to eat a large meal too late in the evening....like in Spain.
But lots of us Brits say lunch and dinner, or (me) dinner and tea (food not drink). I dont know anyone who says supper. It's my understanding that the word 'supper' is more common in Scotland than England and Wales. When i was little, supper was an extra meal late in the evening that was only eaten on rare occasions, but I dont think this use is common south of the (Scottish) border now either.
Not if the English sentence to translate was "What was he eating...", but in general, yes. And frankly, I wouldn't see any change of meaning between "jadł" and "zjadł" here... this is one of those situations when perfective and imperfective don't behave exactly logically, and both variants seem perfectly natural to me.