But you're not going to learn declension and cases from repeating the same few stilted examples that don't even exhibit enough variety to pick up the underlying rules, especially as an adult.
Duolingo is trying to do implicit language learning but the language input is far too narrow.
I used Duolingo to start learning a language with a different alphabet, and it taught me the alphabet, the sounds, and some basic vocabulary. But it couldn't teach me verb conjugation, noun declension, plurals, ownership, etc. etc. etc. That I needed a teacher for.
With the teacher, I then used Anki cards to help with remembering more vocab and with keeping things fresh everyday in between lessons. Duolingo could be that, if they had enough examples, perhaps. I would prefer Duolingo type exercises over my Anki cards, as well as the streak and friendship network effects, but there's simply not enough content.