You could find "des riz" if someone were dealing with several types of rice: long grain, black rice, round-grain rice, etc.
"Des sacs de riz" (bags of rice) would still treat "riz" as a mass, unless the sentence gives some detail about that rice, like "... des sacs du riz qui est ici..." = "... bags of the rice that it here...".
"Un sac de riz" is a "noun of noun" construction, where the second noun, after "de" and without an article, gives further information on the first noun's content, material, or quality.
A dog is eating "some" of a thing, but whether you use "de", "du" or "des" depends on the gender of the thing, and whether the thing is a singular or a plural! So, if the dog eats some cake (le gateau) you would say "du gateau" (because "de le" always contracts to "du"). If the dog eats some banana (la banane) you would say "de la banane". If the dog eats some rabbits (les lapins) you would say "des lapins" (because "de les" always contracts to "des"). Using rice as an example is not very helpful, because there are lots of grains of rice, so you might be tempted to use "des", but even in English we treat rice as singular. You would only use "des" if you were talking specifically about the grains of rice - "des grains".
Sorry, but the "lapins" example is not correct.
"Des" is the plural indefinite article that English does not have. It is distinct from the partitive articles "du/de la", which are used with uncountable nouns (therefore singular).
"Des" is the plural of "un" or "une" and it means "more than one".
Therefore, "le chien mange des lapins" (the dog eats/is eating rabbits) is the plural of "le chien mange un lapin" (the dog is eating/eats a/one rabbit).
Now, when the preposition "de" precedes the definite article "les", they contract to "des". But this "des" is not the plural indefinite article, but the plural contracted definite article:
- Verbs constructed with the preposition "de": Je parle des (de+les) chiens = I am talking of/about the dogs
- Possessive case constructed with the preposition "de": C'est le chien des (de+les) voisins = it is the neighbors' dog (lit. it is the dog of the neighbors).
Once an uncertain amount of a food is mixed together then it is often treated as a singular item (it is called a "mass noun"). So in English we say "I eat a banana" or if we eat several at once, "I eat some bananas". However, if you have a bowlful of mashed bananas (they are in one mass) and you do not know how many bananas are in the bowl, or how many you have eaten by eating some of the contents of the bowl, you say "I eat some banana". It's the same in French - je mange une banane; je mange des bananes; je mange de la banane. Now, if you have to treat bananas as singular when you are not sure how many of them make up the mass of bananas in a bowlful, then you are even less likely to know how many grains of rice are in a bowl - so the rice is treated as a singular quantity as a "mass noun".
Since you do not have any other elements of context, either "eat" or "are eating" is correct, and accepted.
With other information on a time frame, you will have to make a choice:
- En ce moment, les chiens mangent -- are eating
- Habituellement, les chiens mangent -- eat
When the French are willing to be explicit, they use a phrase "être en train de + infinitive" which tells you the action is in progress at the time they speak:
- Les chiens sont en train de manger -- are eating
"Du riz" means "some rice" as "an unknown amount of a mass thing", but it translates to "rice". You don't need to add "some" but if you use a wrong determiner, like "the", the system suggests a replacement of the wrong word with another more correct one, which is "some" in this instance.
"The rice" is the translation for "le riz", specific.
I believe that to be correct (hopefully someone else who knows more will confirm). But in summary, I read someone else's reply (in another thread) that was related to the concept of 'generality' and they said the French use the definitive article 'the' (le/la/les) when referring to something in general. So I imagine "Dogs eat rice" (as in 'all dogs generally') should be "Les chiens mangent du riz".
The awkward bit with French (as someone who's English) is trying to figure out whether that person is referencing dogs as in 'that group of dogs over there that I can see' vs 'dogs in general'
Yes I agree, I think you can figure it out only from context. But in case of generality,wouldn't you say "le riz" instead of "du riz" in your sentence "Les chiens mangent du riz", as I am talking about rice as "a type of food" (le riz) not "some amount of rice" (du riz) ? Hopfully someone will explain the correct way.
If all dogs ate rice, you would say "les chiens mangent du riz", where "les chiens" (dogs) are generalized but not "riz", because they only eat "some" each time.
To generalize "rice", you need something like "le riz est l'aliment préféré des chiens" (rice is dogs' favorite food), where "le riz" is a food category, "l'aliment" is specific and "des chiens" the contraction of "de+les chiens", ie "dogs" as a category.
Also, note that the latter sentence can be said about your own dogs just the same way in French, but with "rice is the dogs' favorite food".