If you keep the mouse over "mangia", where duolingo corrects you, you are able to see a button with the text "conjugate". Pressing it will show you that in Italian verbs have a different form for each person and number, and also, which one you should use for the current case.
Io mangio = I eat
Lui mangia = He eats
Edit: As with the latest updates, the "conjugate" button doesn't seem to exist anymore.
I know the article inclusion happens more often in Italian than in English without affecting meaning. I have a question, though, about how to emphasize the article in Italian. For example we have this sentence which tells us of a boy who eats cheese. Fine and dandy. However, if i want to specify that the boy eats cheese to the exclusion of other foods, possibly fruit, that are on offer as opposed to general cases, in English i could say "the boy eats the cheese, the girl eats the fruit." How would this be expressed in Italian with the specifying articles?