"dei panini" is another partitive in Italian. It can be translated equally correctly into English as "some sandwiches" or simply "sandwiches." Both translations should be accepted in your exercises as correct.
Especially as the program accepts the phrases as correct with an without "some". It should be consistent.
Just as a partitive article in French → des → is the plural of the indefinite articles un / une, dei is a partitive article in Italian and is like the plural of un/una (and, in both languages, des / dei vaguely translates to "some"). Even though the partitive is always required in the Italian and French sentences:
"Lui mangia dei panini" and "Il mange des sandwichs"
it is not required in the English translations: "He eats sandwiches."
Why is this considered incorrect? The boy eats some of the sandwiches?
because dei is not "some of the" but just "some". Your translation puts a different emphasis on the sentence.
How would you write "The boy eats some OF THE sandwiches", rather than only "some"?
I spoke to my Italian friend and she said that the word 'some' is 'alcuni' in Italian and that it should really have been 'Il ragazzo mangia alcuni panini'. And we have been taught that 'dei' equals 'of', so is quite confusing.
So if I say "Mangio i panini" then I eat all the sandwiches that were served.
And if I say "Mangio dei panini" then I eat more than one but not all of them.
Am I right?
I answered "The boy eats the sandwiches". Would the correct translation to my sentence then be "Il ragazzo mangia degli panini"?