Below is a good resource for learning about the use of the "partitive article."

https://www.lawlessfrench.com/grammar/partitive-article/

Bonne chance!

If you are using Duolingo on a web based platform rather than an app, I recommend that you check out the Tips and Notes for Basics 1. There is a lot of good information there and it may help clear up some confusion.

First of all, these all mean "of the"

De l' precedes a noun that starts with a vowel or silent h, e.g. "de l'eau"

De la precedes a noun that is feminine, e.g. "de la natation"

Du precedes a noun that is masculine (you would never say 'de le'), e.g. "du poisson"

Des precedes a (feminine or masculine) plural noun, e.g. "des trottoirs"

Take a look at this page it will clarify it for you and make it easy.http://www.french-linguistics.co.uk/grammar/dudelades.shtml Bonne Chance :)