Does the tree get harder after each level?
One question mark that I currently have about Duolingo is whether the exercises on the tree get harder after each level.
For instance: I'm on English Level 12 now. When I move up to Level 19, will the questions on the tree still be at the same difficulty level, or will they have gotten increasingly more difficult?
You've finished your tree (congratulations!), so advancing in levels is now going to consist of practicing what you've seen and doing translations in the Immersion section if you wish. The lessons themselves shouldn't get harder. However, advancing in level will take more points. From Level 19 to Level 20 is about 1500 points, and it goes up after that. Good luck.