This question has bothered me for a while. When I learned french long time ago, our teacher said: "l'article partitif means some". Example: J'ai mangé du fromage -> I ate some cheese, j'ai bu de la bière -> I drank some beer.
Later, we have learned that when put l'article partitif in front of an adjective, it often maintain "de". Example: J'ai de bons amis -> I have some good friends. In this case, we don't say "j'ai des bons amis".
But here comes the problem, in one exercise there are 2 sentences: "j'ai bu du bon vin" and "J'ai bu de bons vins", and they both are right. I am confused, what is the difference between these 2 sentences and why we put "du" in front of "bon"?
thanks