I am still amazed, almost every day, by the difference between learning something for a test and actually understanding it. Every year, I try to cover less and convey more. Professors often want to max what they COVER, but what you should focus on is what students learn. I bet some years I cover a lot, and the kids learn almost nothing. It's humbling, and I need to do better. And I've already been doing this for more than 40 years!