The so-called omniscience is only the first level, and the purpose of knowing is to solve problems. So doing exercises requires a second level of proficiency. At the third level, you can not only do exercises, but also do an exercise and express it fluently. This requires memorizing some basic theorems and formulas, as well as some common problem-solving methods. As a teacher, we must reach this third level.

Just as you can understand an article, it doesn't mean that you can fully understand it, let alone write such an article.

Only through practice can we achieve real understanding and familiarity.