Up: To produce a shot with little distance or power.
Right: To hit the ball to the right side of the screen.
Down: To produce a shot with long distance and power.
Left: To hit the ball to the left side of the screen.
You can also use combinations such as: Left + Down, etc. This information has been submitted by HRBEK
You will find it much easier to return opposition serves if you stand up closer to the net. This greatly increases the chance of you returning the serve. Against players who serve slower, you will not have to resort to doing this.