Leaving app restarts my lesson
This has happened 3 or 4 times now: I leave the app partway through a lesson (dative case 2/3) while I'm writing a comment on a question, and when I reopen it, it's quit the lesson and I have to restart. This happens whenever I'm in the comments section for a question, and then switch to another app. So annoying.
So our devs looked into it and this seems to be a problem on Google's end. See: https://code.google.com/p/android/issues/detail?id=26658
Until they can fix this, we'd suggest just restarting your device and opening the app from your app list and not from the Play Store.