Why is it "du" and not "des"? I don't get it.
Because here there is no plural. We are talking about an undefined quantity of rice, hence "du riz".
You never say "des riz".
Why then "des poissons" as a similar example given earlier? How would "fromage" be used then?
Isn't 'are eating' a different verb tense than mangent? Isn't mangent 'they eat'? Maybe these are the same tenses? They eat, or they are eating. Subtle difference, I think.
In French, there's only one present tense. In English there's three. In English you could say "I'm eating", "I eat", or "I do eat". In French, all three are the same, "Je mange".
why is it du riz and not le riz?
noooo ducks dont eat rice