The biggest mistakes on the forward swing are actually taught mistakes. Many teaching pro’s will teach players to move their wrist before contacting the ball. They will also teach players to stay low as the racquet is moving forward.
The biggest mistake on the forward swing is moving the wrist before contacting the ball. In some cases this is taught (many teaching pro’s teach this as part of the modern forehand), in other cases it is what players naturally will do and still, in other cases, it is done out of necessity as otherwise the player would not be able to hit the ball.
Many players will use their wrist to get the racquet vertical early into the swing thinking this needs to be done in order to hit the ball.
Make sure to keep the wrist locked. The racquet will eventually get vertical automatically as the racquet and arm swing forward.
With a correct forward swing a player cannot afford to be late hitting the ball as the racquet face will be closed until right before the correct contact point is reached.
Players can afford to be early to the intended contact point with a correct forward swing, but they cannot afford to be late. There are ways to still hit the ball when late without moving the wrist, but this is much easier said then done. It is easiest to just not be late.
Another big mistake is not starting to unbend the knees to lift the ball up. In some cases this is also taught. Many coaches will tell players to “stay down” through the shot. The knees must start to unbend as the rest of the body is lifting in order to keep the body and racquet in unison with each other. Since the arms, upper body and racquet are lifting up, the legs need to too. This also will help in aiding the recovery process later into the swing.
A third big mistake on the forward swing is not decelerating the left arm and getting it too far away from the right.
The left arm will initially move further away from the right, but by the time the racquet is about to hit the ball the left arm should have decelerated substantially to help the kinetic energy transfer into the racquet. If the left arm is not decelerated the racquet will not be moving as fast as it should be at contact. This will also most likely make the swing horizontal which leads to a potential adverse effect a little later into the swing.
The fourth big mistake made on the forward swing is not keeping the head still. The brain thinks the racquet hits the ball before it actually does so many players will lift the head up early to see where the ball is going. The head is the heaviest part of the body so any movement of the head will move the center of gravity. As the head moves up to look it will move the racquet too. A sure sign of early head movement is the ball hitting off the frame.
A fifth big mistake is jumping into the shot before the ball is hit. Jumping too early can lead to bad balance by either pulling the body too far forward which will move the bodies center of gravity too much (destabilizing the body) or by pulling the body too far to one side (usually to the left). Pulling the body too far to one side will also move the center of gravity too much and have the further problem of making the racquet swing end up too horizontal. Jumping also can lead to needing to use mostly the arm as it can break the kinetic energy chain which can keep it from flowing correctly. Jumping may happen, but should only happen after the ball has been hit as a product of the bodies momentum.