Learning systems like duoLingo don't ship with a textbook to introduce new material. As a result, your first time into a new section will always feel like a pre-test. If you always passed the pre-test 100%, you wouldn't need duoLingo.
Personally, I never count on getting all the hearts during my first pass-through; though I work hard to get all the hearts afterward. I take the emotion that accompanies heart loss, and try to invest it into strengthening the memory.
Sorry to rock the boat. Yes, "till" is where you store money and you can "till" the soil. However. "till" as a synonym of "until" is spelled with 2 ts and it is the original word not an abbreviation of until. Just, the opposite first came "till" then "until". "Til" is Old English and Old Norse of course it might also be regional.
Just google "till origin".
Edit I should have written "...is spelled with two Ls...".