"The boys eat rice."
Translation:Les garçons mangent du riz.
It means an unspecified amount of rice.
Nouns in French need a determiner. The "du" here is a determiner called a "partitive article", used for uncountable things that have a masculine gender. This article does not exist in English, but is required in French. Because nouns need a determiner.
Because the subject is plural. (and in third person)
- le garçon mange → il mange (singular)
- les garçons mangent → ils mangent (plural)
Since "du" is de + le, "du le" is never correct.
The partitive articles in French are used for unspecified amount of objects.
- du = masculine - "du riz"
- de la = feminine - "de la salade"
- de l' -- in front of a vowel or mute H -- "de l'huile"