I a not sure how the hearts on the lessons work so I can progress to the next lesson. In fact as an elderly person it seems a bit confusing and I think I keep going over old ground.
If you lose all the hearts on a unit (3 or sometimes 4), then if you make one more error you get kicked out of the unit and have to start over. But finishing with zero hearts is okay. You get ten points, no bonus points, but you have successfully completed that unit. You can redo a unit to get more hearts, which can be good review, but that isn't required.