Noether's Theorem:
https://en.wikipedia.org/wiki/Noether%27s\_theorem
https://math.ucr.edu/home/baez/noether.html
Fundamentally, it allows us to logically infer the conservation laws from the laws of motion of a given physical system using relatively simple math. It always applies, no matter if we're talking about massive systems or quantum ones.
I remember it, but I cheated by getting cancer that year.