They often don't specify. If you really wanted to specify that something is definite, they put "this" or "that" in Polish, from what I read in another discussion. Demonstrative pronouns are used in Polish.
Check the information given by br0d4 in the following discussion: https://www.duolingo.com/comment/12187035$comment_id=12218732
Not quite, "dz" is pronounced "dz", but "dź" is pronounced "j", "cz" is pronounced like English "ch" which is not the throaty k like sound, but closer to "tsh", "w" is pronounced "v". Scroll down for the tips and notes about the Polish alphabet sounds with IPA on this page: https://www.duolingo.com/skill/pl/Phrases
Yes, it's a correct sentence. The question is whether at this stage (I see you have only just started) you know why it is correct ;) Although maybe you learned somewhere else earlier, I don't know.
Anyway, "jeść" takes Accusative case. For neuter nouns like "jabłko", Accusative is the same as the basic, Nominative form. But if you chose the word "kanapka" (a sandwich), which is feminine, it would be "Dziewczynka je kanapkę". And with masculine Accusative it's even more complicated.
I thought "je" was THEM now its EATS... What is it them or eats. GOOGLE tells you one thing and someone else tells you something different... This happens all the time. One language site tells you one word and another tells you something different. How are you supposed to learn anything when all the different sites have different meanings for the same word, yet they all say they are Polish.
It can mean both. Or rather, I'd say that these are two different words that have one form identical.
"je" is 3rd person of 'jeść' (to eat), ergo: he/she/it is eating/eats.
"je" is also an Accusative form of "one" (the form of 'they' used when there are no men among 'them', so generally when 'they' are only women). "Widzę je" = I see them.
This is very reminiscent of "woman" in Russian. Is there a connection?