Partitive article

Question

Is there any particular reason why "des poissons sur du papier" is correct, but not "du poisson sur des papiers" Thanks!

Céline Pickard · Accepted Answer

Bonjour Tuhin,
If you say/write "du poisson sur des papiers", even though grammatically fine, it means "some fish (i.e. meat) on some documents" which is a strange image in the context of the exercise.
Je mange du poisson = I am eating some fish
-> "du" = partitive article
Je dessine des poissons sur du papier = I am drawing some fish on (some) paper
-> "des" = indefinite article + "du" = partitive article
Je dessine du poisson sur des papiers = I am drawing some fish on (some) documents
-> this would sound very strange in French + "papiers"
You could also say something like:
Je dessine des poissons sur une feuille de papier = I am drawing some fish on a piece of paper
 
I hope this is helpful.
Bonne journée !