@PetraSchwer

I don't know what it means to understand a piece of mathematics.

But I tell my students that if they don't find any mistakes in my lecture notes, this is a symptom that they don't understand the material. To understand it, they must find the mistakes and be able to fix them on their own. 🙂

(I use GitLab for the lecture material, and the students are encouraged to do pull requests to fix mistakes in the the lecture notes, and they do.)