I wrote "the boy eats the apple" and got it right. Can I assume the article "a/an" and "the" are indistinguishable in Polish? Also, are "eats" and "is eating" interchangeable in Polish? (forgive me for now knowing the syntactical/grammatical term)
Indeed, however, "the boy" can also be translated as "ten/tamten chłopiec (suggests a child)/chłopak", the same way that "the boy" can also mean "this/that boy" (which would be the direct translation of "ten/tamten chłopiec)
Just because there are no articles in Polish, doesn't mean you can omit them in English translations in places where there should be one. Duolingo should let you pick any articles that are gramatically correct.
It should have worked, it's accepted. Well, it would show a message about a missing space or something ("A boy is" is accepted, I had to add the version with the apostrophe now), but it should have been accepted.