"They eat sugar."
Translation:Loro mangiano lo zucchero.
Even though your post is 4 years old. For new people that come across this, definite articles are as follows:
il – masculine, singular, e.g. il libro (the book)
i – masculine, plural, e.g. i libri (the books)
la – feminine, singular, e.g. la porta (the door)
le – feminine, plural, e.g. le porte (the doors)
le – feminine, plural, begins with vowel. e.g. le università (the universities)
lo – masculine, singular, begins with s+consonant or GN, PN, PS, X, Y, Z. e.g. lo schermo (the screen)
gli - masculine, plural, begins with s+consonant or GN, PN, PS, X, Y, Z. e.g. gli schermi (the screens)
gli - masculine, plural, begins with vowel e.g. gli anni (the years)
l’ – masculine or feminine, singular, begins with vowel e.g. l’anno (the year)
where Masculine words typically end in the letter o and Feminine words end in the letter a, there are many exceptions to this but don't worry about it for now.
In this case "zucchero" is Masculine, singular and starts with the letter z, therefore it's "lo".
It's more a question of what category of sugar you are talking about. If it's a general category, ie sugar in general, then it's 'lo zucchero'. If it's a partitive category, ie the word 'some' is meant, whether stated or not,
then it's 'dello zucchero'. I believe that you can get away without using any article at all (though you can't in French, which is dead keen on them; Italian may be slightly less so).