Streak: Also let unfinished lessons count
it is somewhat annoying that the streak only continues if I completely finish a lesson. Sometimes I do 10 lessons without being able to complete it because I lose all my hearts. And I really don't want to do a simple lessons like 'Places' just for the sake of keeping the streak up.
Can't you make uncompleted (not canceled!) lessons count? For example, if throughout the day I get 20 correct answers the streak continues (that is more correct answers than in one complete lesson!).
So try an earlier question which you are more confident with. The reason you have to finish a lesson is because the scoring system gives 10 points for completing the lesson plus one point for each life remaining. If you removed that simple algorithm, you would have to calculate a number of points to award depending on how much of the lesson that had been completed and that would complicate the algorithm considerably.