This is what I said understand text books.

"一朋友的同学在国内读物理本科时，把教科书上的公式都自己推导一遍。他还真的发现有一个公式是错误的，找到相关的实验数据，证实了他的推导结果".

if you reach the level that you couldn't understand text books, then just to remember the higher level concepts and practice a lot to know how to use it, so you will  get high GPA in school.