I'm having a hard time finding a definitive reference for a simple question: how do partitive articles behave before adjectives? Most resources mention that des becomes de, but it's unclear to me what happens in the singular, e.g. Je veux du bon vin, or je veux de bon vin?
There are two cases in which the partitive article changes from the default “du”/“de la”/“de l'”/“des” to “de”/“d'”.
With a plural noun, when there is an adjective before the noun. There is no such phenomenon in with a singular noun.
J'ai bu de bons vins. (I have drunk some good wines.)
J'ai bu des vins excellents. (I have drunk some excellent wines.)
J'ai bu du bon vin. (I have drunk some good wine.)
By the way, “des bons vins” is also grammatically correct, but it is the plural of the indefinite article: “un bon vin” → “des bons vins” (a good wine → good wines), “du bon vin” → “de bons vins” (some good wine → some good wines). In French, like in English, there are many contexts where either a plural indefinite article or a plural partitive article could work, and the similarity of the articles contributes to the fuzziness.
When the noun phrase is a direct complement of a negated verb.
Je n'ai pas bu de bon vin. (I have not drunk any good wine.)
Je n'ai pas bu de bons vins. (I have not drunk any good wines.)
Je n'ai pas bu de vins excellents. (I have not drunk any excellent wines.)
I found this post when researching the same thing, I found this that does explain that
de before adjectives [before nouns] (it's at the end of section 2). Cheers!