1/2 = (1 * 8) / (2 * 8) = 8/16
3/16 - 1/2 = 3/16 - 8/16 = -5/16
If you're someone who wants to visualize and have a more intuitive understanding of this, imagine a large pizza cut into 16 slices, and you have 3 slices left. Suppose your friend calls you and says "hey, I'm super hungry, and I'm going to eat half a pizza", do you have enough pizza left for him? And if not, how much are you short?
Then out of a whole 16 slice pizza, half would be 8 slices. Since you only have 3 slices, you would be 5 slices short, so your friend should pick up more pizza before he comes over.