Another user pointed out some poor phrasing in a French sentence I had linked to by saying "C'est du mauvais français."
I am not sure why this uses du rather than de; my impression was that the adjective would cause you to use de. I still oftentimes confuse the proper usage of de/du/de la/de l'/des in cases like this one.
The correct usage would be "C'est du mauvais français", not "de". "français" is here used as something quantifiable and definite (de le => du français, de la langue française, des mots en français, that you can touch and split like du pain), not something abstract like an adverb describing a manner (e.g. c'est de bonne heure = it's early, c'est de bonne guerre = it's fair enough)
Distinction is made (sometimes not easily) between the quantifyable noun and the abstract notion as an adverb. Example given, "c'est de bonne guerre" meaning it's only fair, and "c'est de la bonne guerre" meaning this is some good war.
You are not constrained to use "de" with an adjective + noun, but more often with the adverbial clause.
Le français = la langue française. (l'anglais = la langue anglaise).
this is singular => du (bon / mauvais) français = de la (bonne / mauvaise) langue française.
du/de la/de = partitif/indénombrable (matière, liquide): du lait, de bons sentiments.
de mauvais français = de mauvaises personnes françaises.
On dirait ce sont de mauvais français (pluriel => ce sont)
It's a rule in French, the construct "de le" doesn't exist and is replaced by "du". As opposed to the feminine for which it is "de la".
Je mange de la glace. I eat ice cream
Je mange du pain I eat bread
Je me souviens de la mer. I remember the sea
Je parle du français. I'm talking about French
