The rule is " the tu and voi forms are identical to their corresponding present indicative forms, except for the tu form of -are verbs, which add-a to the root: domandare > domanda", taken from
I have a guess that maybe the "lo" before the zucchero makes it to be a general statement, and that is why it can't be imperative(?), so "Agguingi zucchero al tè" is the imperative translation? Google translate suggests the opposite ("Agguingi zucchero al tè" - present simple, "Agguingi lo zucchero al tè" -imperative), but I don't know if it is reliable with grammar issues...
Most of the time, the presence of the definite article in the Italian doesn't seem to necessitate the same in English. But in this case "You add sugar to tea." was marked incorrect. Can anyone explain why the definite article, in this particular case, indicates "this particular tea" and not "tea in general"?