When you slice the ball you are putting backwards spin on it. It's easier for most beginners to hit a backhand slice because the motion is more natural.
When hitting a backhand slice you can open your stance more...meaning your belt-buckle can be facing the net. With a forehand slice you need to stand more perpedicular to the net...meaning your belt-buckle would be facing the side-line. As you hit the shot, keep your toes facing the side-line but alow your trunk to turn so that your belt-buckle is turning back toward the net
Second, you might notice that you end up slicing the ball accidentally when you have to make a long reach for a low ball. This is because the point of contact on this shot is way out in front of your body. So, to hit with slice (intentionally) think about making earlier contact with the ball--in front of your body.
Thirdly--on a forehand slice you need to keep the ball further out away from your body, than on a backhand slice (to give your arm enough range of motion).
Sooo...start with your toes and trunk pointed to the sideline in a base-ball player stance; reach out to make contact with the ball further in front of you and a little further away from your body; and as you make your slicing motion turn your trunk back toward the net, but keep your feet relatively still.
Let me know if that helps.
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