“Ah, I remember now!”
But do you really?
The human brain can recognise a familiar face in a fraction of a second. But have you ever tried drawing a familiar face from memory?
Suddenly a difference between merely recognising and actually remembering appears. In school this can be an insidious trap: Until the exam comes it may seem as if everything is understood because all facts and formulas feel familiar. But only by actually trying to apply them is it possible to find out of this is true knowledge or mere recognition.
How to avoid this trap? Learn actively!
Instead of looking at a page of formulas and trying to learn them by heart, start with a blank page and try to write down as many things about the subject as you can without looking at your notes.
Do exercises and practice problems that are as close to those in the exam. Add a challenge by not even looking at the formula collection. Or by re-deriving the formulas from scratch.