"How many red apples do you eat?"
Translation:Quante mele rosse mangi?
I haven't learned whether all of the subject pronouns are optional in Italian, but some are.
The point is that at least some of the singular and plural forms of verbs have a specific ending that includes the meaning of the subject pronoun. Thus, = "I am" = "sono," "all of you = "voi siete," "è" = "he, she, or it is," "we are" = "siamo," and "they are" = "loro sono." "Mangi/eat" is the singular form of "eat," and doesn't require the subject pronoun "tu/you."
Voi mangiate = all of you eat, which is plural.
Tu mangi = you eat, which is singular.
The reason why "Voi mangiate" wasn't accepted is because not enough people have hit the "My answer is correct" button. When enough people report that "Voi mangiate" should be accepted. it will be listed under "Another correct translation:".
Imagine you have this question: write down one fruit, so you write an apple. After that someone else writes a pear, but for some reason "a pear" is not accepted, while "an apple" is. After asking why one is accepted and the other isn't, even though both answers are correct, and you say - it will be accepted when enough people say that pear is a fruit...
This is how your answer, to Susan's question, looks like here, Linda.
It may not be the answer susan was looking for, but its the truest answer. Susan's answer was a correct way to translate the sentence, but is not accepted by duo because duo only starts with one "correct" answer, and extras have to be added in by people reporting them. Until enough people report the same correct answer, it will not be accepted even though it is "correct." what would you prefer to tell people in Susan's situation?