This is where understand WHAT f-stop is, HOW is works, and WHAT it does to the image. Same goes for shutter-speed, along with your lens length, and such. One you understand what is going on (in terms of the image as the end result) you will KNOW what to do, and WHY you are doing it.... Exosure is only ONE (small, albeit important) part of the puzzle.
wait, did that make sense?