Basic math, such as... Deriving the quadratic equation, taking the derivative of a logarithm in a base other than e, taking integrals of trigonometric substitutions, remembering what a subspace actually means, not remembering how to derive the general solution for a first order differential equation, etc..? Of course, it's much quicker to do those things when you look back at them again (and you don't have to struggle to understand anything anymore). But still, sometimes I worry about my memory problems or something. Is it even common for them to sometimes forget them multiple times? Like, forgetting to do it once, then forgetting to do it 4 years later?