The problem is that you’re grinding something you already know (in my experience always something too easy) just for hearts, and the heart system removes the ability to grind something you don’t know until you know it in a lesson. I want to make mistakes until I nail it when I’m learning.

That said, at least the Pro version fixes it.

Thank you Captain Obvious. It is still useless grinding that takes time and gives nothing to learning.

Oh, and you start being afraid to make mistakes, to not be forced to perform said grinding over and over again.

I have 800+ days in Duolingo. There were no hearts when i started, it appeared when i was in early 700s and I learned how to switch them off with classroom trick. If I had hearts from the start, I'd choose a different app. Hearts are that bad.