"I am eating rice."
Translation:Je mange du riz.
There's a grammatical difference between French and English when it comes to uncountable things. Basically you wouldn't say "I am eating apple", you would say "I am eating an apple". The same thing applies in French, but it applies to things that are uncountable as well. "Some rice." "Some wine." That sort of thing.
Du = de + le. It's not plural! Plural would be des = de + les. It means you are eating some rice, not THE (certain amount of/certain) rice or A (only one) rice. Un riz is a rice and le riz is the rice. The preposition de is used (beside many other situations) when you want to say you are eating some of the cake (du gâteau), not the whole cake (le gâteau). You can also order something in a restaurant with de: Je prends du gâteau, s'il vous plaît = I take cake, please. You wouldn't just go and order THE cake or ONE whole cake, right? :)
French would probably think you are trying to say: J'ai mangé, which means: I have eaten. French use this instead of I ate. As said above, je mange means I am eating and I eat. If you want to highlight the fact, that something is happening now, say je mange maintenant (=I eat now) or je suis en train de manger. This literally means: I am eating. The construction is: Être en train de+inf. of the main verb. Être must be conjugated, je suis, tu es, il est...