du and le/la
What's the difference between du and le/la? When do i say "je mange du riz" and when do i say "je mange le riz"?
This might help http://french.about.com/od/grammar/a/de-vs-du-de-la-des.htm, especially the first point
1) The definite articles (le, la, les) are used with nouns that represent beings or objects considered to be known, or that have been previously introduced into the verbal or written narrative. “Il a acheté la table chez un antiquaire.” The use of “... la table ...” implies there is a specific table under discussion, as opposed to “... une table ...” meaning some table we may not have seen or know much about (other than that it is probably an antique).
2) The partitive articles “du” (de + le) and “de la” are used to indicate that we are dealing with a certain quantity of a product (powder, liquid, pâté ...) that does not constitute a collection of objects than can be isolated. So: “Il mange du pain.” At least grammatically, this means he is eating part of a loaf of bread. Remember that French bread is usually sold unsliced, so it is infinitely divisible! For “Il mange le pain.” see 1) definite articles. If you say: “Il mange un pain.” he is probably eating a bread roll.
3) To answer docsimsim’s question about which is more commonly used? For objects that cannot be isolated (flour, bread, rice, milk, etc.) you can use either, but it all depends exactly what you mean. “Il a mangé du sucre.” This just means he ate some sugar. “Il a mangé le sucre.” This means he ate all of the available sugar under discussion. “Il a commandé du vin.” He ordered some wine. “Il a commandé le vin.” He ordered the particular wine we were talking about earlier.
4) Points 1) to 3) deal with the grammatical usage of the definite and partitive articles within a narrative context. A further complication arises in French because you cannot use a noun without the definitive article. So you cannot talk about the product rice as “riz”, you have to say “le riz”. This may mean a specific, known quantity of rice, just as in English usage: “pass me the rice, please”. On the other hand, it may mean rice in general. “Je mange le riz.” could mean I am eating all the rice, or it could mean I do eat rice from time to time. You can see from 3) above that when using the verb “manger” in the past tense, it is very clear what is meant, but not in the present tense. Duolingo really doesn’t help matters at all, by presenting these sentences out of context when attempting to teach use of the definite article.