The practice lessons give you back hearts, 1 per practice plus another 1 if you watch a short ad afterwards. Starting a new lesson will sometimes also give you

The practice lessons give you back hearts, 1 per practice plus another 1 if you watch a short ad afterwards. Starting a new lesson will sometimes also give you an opportunity to watch another ad for a new heart, so after doing 2-5 practice lessons you can go back to progressing the new material you were working on before. I actually prefer the free version, because it naturally creates spaced repetition by forcing me to review old lessons in-between new content.