I was learning Serbian language recently and did the same mistike xD

Just what I thought! I was learning Swedish before and I got so confused when I saw this...

*Nordic speakers

OK, I guess we can add "Right".

"Tak, on je jabłko" - "Yes, he is eating an apple" (just any apple). But also, if in previous sentence there was something about some apple, we may assume that it is the same apple: "Yes, he is eating the apple" (that we spoke about). You may also stress it more, using a pronoun "to" (the mentioned, this, this one - singular, neutral gender) or "tamto" (that, the other one - also singular, neutral gender): "Tak, on je to jabłko" - would be "Yes, he is eating the apple" or "Yes, he is eating this apple" (that we spoke about).

There is a vast range of pronouns in Polish. In this very case, the demonstrative pronouns are to be used. See also (general) Polish_grammar#Pronouns ; and (detailed): Polish_pronouns or Język_polski_-_zaimki

Also, please note, that in Polish there is no formal distinction between Present Continuous and Simple Present. "Tak, on je jabłka" - "Yes, he is eating apples" (now), "Yes, he eats the apples" (he eats every day the apples that are stored in his pantry), or depending on context may mean "Yes, he eats apples" (but not pears).

The same way for uncountable "Tak, on je ryż" - "Yes, he is eating rice" / "Yes, he eats rice".

I hope I helped. I am not sure If I put it clearly in English...