"I will teach the stupid professor."
Translation:저는 머리가 나쁜 교수를 가르칠 것입니다.
I can't imagine how you changed 나쁜 (nappeun) into "napunan." "Nappeun an" would mean "bad an," whatever "an" is, but "bad" is an adjective made my adding -n to the root of the adjectival verb. Then "mori-ga" would be the subject of a predicate, so "meoli-ga nappeu-da" "(somebody's) head is bad" makes sense. Change -da to -n and you've got "bad headed" or "whose head is bad."
People are more likely to remember something from a mistake than from a lucky guess that was right. But, being penalized in game-terms is also demotivating. To meet both needs, the game would need to stop penalizing wrong answers when it's your first time seeing the material. That also makes Duolingo more complicated, so that will just take time as they slowly improve the incentives-system.