"We are eating rice."
Translation:Nous mangeons du riz.
"le riz" is a masculine noun.
Eating rice is about eating "an unknown quantity of a mass thing" or "some rice".
This translates to a partitive article: "du riz" ("du" is the contraction of the preposition "de" + the definite article "le").
to anyone wondering why it is mangeons and not mangons, it is a special exception to this conjugation.
The reason is about pronounciation of "G" in front of a O (or A, U) which is a hard G (like the first one in the English "garage") vs in front of a E or I, which is a soft G (like the second one in the English "garage").
So, an "E" is added between G and ONS to keep the soft G pronounciation.
The -e- is added to keep the soft sound of the G in front of a hard vowel (o).