What should you do if you hurt your ankle about six months ago and the doctor who examined it said it was a sprain but it still hurts and does not feel right and cracks constantly when walking?

Head back to the same doctor and tell them it's not fixed, and require them to work it up again. X Rays, MRI if needs -- whatever. While sprains can take a long time to heal, it's important you're comfortable with the direction your therapy is taking -- and clearly you have reason not to be. The cracking could be a sign of a common form of Arthritis, depending on if you had that before the injury. If the problem hasn't gone away, you never gave it enough time to heal. And just a tip - sometimes its better to not listen to the doctors. If you are back playing sports or whatever and it still hurts, take it easy until you can perform comfortably without pain.