Lay vs. Lie

I just can't keep it straight. No matter having a language arts teacher for a mother, no matter having looked it up online 10 times, no matter having asked this or that person over and over again. I cannot remember when to use "lay" or "lie" or "lays" or "laid" or "lain." Can't do it. 

A language is an infinitely vast and flexible thing - why then must we use the same words to mean multiple things?! Ahhhhhhhh!