because in english the pizza can be referring to the whole pizza or a single slice, and the milk and milk refer to milk as a whole, we don't have the singular or plural her unless we mention a bottle of milk or a slice of pizza specifically. Which is a difference scenario with candy.
I can guarantee this isn't duolingo specific, because I have absolutely no problem with that, probably because Portuguese is my native language and it is quite similar to Italian in this aspect. However I don't know if I know enough Italian to give a proper explanation about the articles... Just telling you this isn't about Duolingo.
I disagree. Candy can be plural or singular, regardless of the determiner.
In fact, WordRef (https://tinyurl.com/y677l3tw) translates "Le caramelle fanno malissimo ai denti." as "Sweets are really bad for your teeth." They don't include the determiner. Similarly, "he eats candy" and "he eats the candy" can be equivalent translations (except here we might infer that it's either singular or referring to a class of food called candy).
Being English I put "he eats the sweet" which was marked incorrect, should be (American) "candy", then I was asked the question again and again I said " he eats the sweet" and now it is marked correct with "the candy" as an alternative. How inconsistent is that? Same answer incorrect then correct....
Actually, one can say "the candy" in the US. Example, "Why does Ben's stomach hurt him? - He ate the candy." (Implies that both people in the conversation know to which candy they are referring, as opposed to candy in general. "The candy" could be the Halloween candy that had been sitting in a bowl that's now empty.
The problem is that until now I've been trained to ignore the articles by duolingo, because Italian overuses articles compared to English.
It's a known problem of Italians speaking English that they put "the" before every noun. So when I see "la caramella" I've been trained to just read candy. Like mi piace il latte would be I like milk, io mangio la caramella can be interpreted as I eat candy.
There has to be a better explanation somewhere of when to ignore articles and when not.
All of you who are having "sweets" marked incorrectly should report it, if it will let you. There's usually a button on the page where it says you're wrong, that you can click on and report that the answer should be accepted. If enough people do that, perhaps Duo will start to accept that as an alternative answer.