The position of the ball is second only to the grip, and mistakes in grip and ball position are mistakes made before the swing that may ruin any thoughts of a good swing even before you've swung. Many golf coaches teach that the ball should be played off the left heel for all shots however, I'm not a fan of this doctrine as only very good players can do this off good lies. But if you play the ball off your left heel with a 9-iron, you must have a very fast hip shift to meet the club with the ball on the downswing.
The driver and a teed up 3-wood are the only clubs you want to play off the left heel. This is because you want to hit the ball slightly on the upswing or at the lowest point of the swing with these clubs. With the other clubs move it back a fraction of an inch at a time until you reach the centre -- where a nine iron belongs. If you have any doubt about where to position the ball for any iron, take a few practice swings and note where the clubface brushes the ground.
Out of Bounds: If you're not sure which part of the clubface is actually striking the ball, whether it's the putter, an iron shot or a driver. You can easily find out by simply taking some talcum powder with you to the range or putting green. Powder the ball. Hit it and look at the clubface.