For my fellow mobile users: there are actually rules in the desktop version of Duolingo. You just have to open the lesson from the site, the grammar is at the bottom.
Can someone please help me understand why it is "Enn grijze muis", but it's "Een zwart boek"?
Adjectives before het-words (like boek), don't end on -e.
De zwarte muis
Een zwarte muis
Het zwarte boek
Een zwart boek
Is there an "easy" way to remember which words are Het and which are De? It seems pretty random to me.