If you are hitting the ball high in the air, you are probably too close to the ball and hitting it on the side of you, rather than in front. Try to hit the ball in front and keep the ball about half a metre to the side of you.
A good little test for this, is to make sure that you could fit a tennis ball under your armpit when you hit the shot. Your arm should not be very close to your body. If you are hitting the ball sideways, you are definitely hitting the ball too late - again, try to hit the ball in front of you.
If you are hitting the ball into the bottom of the net, or sometimes even bouncing it before it goes into the net, you are probably swinging down onto the ball rather than lifting the ball 'out of the water'.
If you are hitting the ball hard and flat, but with no control, you are probably slapping the ball with your wrist, more or less like a squash shot. Slow the swing down and just rally at a medium pace, until you get control of the ball.
With any of the errors above, make sure that you try to hit the ball in front of your body and see the ball hit the strings on the racket.
If you are skying the ball, make sure that you are hitting the ball in front of your body and that the racket face is not bending under the ball, to face the sky, as you make contact.
If you are hitting the ball into the net, or bouncing it onto the ground before it reaches the net, make sure the racket face is not turning to face the ground, as you make contact with the ball. It is very important on both forehand and backhand, to learn to control what the racket face is doing. See the video with smiley face, to understand exactly what you want the racket face to be doing.
Practice progressions for ground strokes - remember 'CSS', control first, spin second and speed third. Until you can control the ball with some spin, there is no point in trying to hit the ball hard. It is actually harder to learn to control the ball, at a rallying speed that you are comfortable with, than it is just to slap balls all over the place. Get the control first, get control of your spin second and then think about speed. Learn how to rally 100 balls in a row, down the middle of the court, without missing.
Then progress to 50 balls cross-court on the forehand and 50 balls cross-court on the backhand, without missing. The next progression would be one player hitting the ball down the lines, the other player hitting the ball cross-court, so you are both moving up and down the court at a controllable speed. Again, try to reach 50 balls.
Once you can do this, you already have excellent control, spin and direction. As you progress with this, slowly but surely, you will be able to hit the ball harder and harder in these exercises. Do not try for speed first.
by David Sammel