Pro-tip: Hearts only exist in the app, not on the website.
As soon as they switched to the hearts system, I uninstalled the app and now exclusively use the mobile website. Considering their app is just a web view, the experience is essentially the same (you'll need connectivity at the start and end of a lesson), except no ads and no hearts!
As a plus, the website actually allows you to view the forum discussion on sentences, for example if your answer was wrong but you feel it should be correct, you can see what other people are saying about it. Why they don't offer this in the app is beyond me.
The decision to gate people's learning by restricting their number of attempts stupid. It's counter to the point of learning, but I guess they didn't feel they were getting enough revenue from just ads alone, so they have to kneecap the learning experience to force people into their over-priced monthly subscription.