What is the most useful concepts of programming you've learned?

Don't touch if it works.

are you blind?

binary / bitwise logic
oop paradigm (this one gets you a job)
memoization & recursion

The proper use of the ternary operator.

continuations and monads

based

lambdas

Factoring

also this

recursion
data structures
lambdas
OOP and Functional paradigms

That you can tweak C to perform hardware-aware operations without using the asm voodoo.

i can never get recursion no matter how hard i try

I studied dozens of data structures but only use four of them at work lol. I think design patterns are useful -- just don't adhere to them as you would a religion.