how do the hearts work?
Hearts are like "free cards" when practicing or working on a lesson. If you have 3 hearts showing, you can make 3 mistakes before you are on the verge of not passing the lesson (or practice session). I say "on the verge" because you can still finish a lesson with no hearts left; you just need to make sure you don't make any more mistakes. So, to be clear: 3 hearts means 4 mistakes and the lesson is failed. 4 hearts means 5 mistakes and the lesson is failed. However, the best part is, for every heart you have left over after a lesson or practice, you get a bonus skill point :-).
I started Duolingo with three hearts, and as I progressed a fourth heart opened up. It's suddenly gone today despite doing a lot of work and passing a lot of lessons, levelling up, etc. Do you have any idea where my fourth heart went, or how I can get it back?