(Note - this is an expanded version from a previous blog post.)
Let's do a thought experiment. Hold a piece of paper so you hold the top with one hand, the bottom with the other. Now twist the top. Notice how the entire piece of paper twists? Now twist the bottom. Same thing. How does this relate to table tennis?
Now imagine holding a table tennis player in your hands. (You are either very strong or the player is very small.) Hold his playing hand in one hand and his feet with the other. Twist his playing hand and his entire body twists. The same if you twist his feet.
This is what happens when you have a bad grip or foot positioning. It twists your entire body out of proper alignment, and are the most common cause of technique problems. Most often they are not recognized as even many experienced coaches often treat the symptoms of these problems rather than recognizing the cause.
This is why I strongly recommend that players should use a neutral grip (along with a proper stance) during their formative years, and usually well beyond that. (For shakehand players, a neutral grip means the thinnest part of the wrist lines up with the racket. If the top is tilted away from you when you hold the racket in front of you, it's a backhand grip. If the top is tilted toward you, it's a forehand grip.) A neutral grip means your racket will aim in the same direction as your body is stroking the ball. A non-neutral grip forces you to adjust your stroke in often awkward ways since the racket is aiming one way while your wrist, arm, shoulder, etc., are aiming another direction. This can lead to many problems.
For example, a forehand grip often leads to an overly wristy forehand loop, which makes it difficult to learn to control the ball, and also makes counterlooping and looping in general against fast incoming balls tricky. It can force the back shoulder down in an awkward attempt to adjust for the naturally overly closed racket angle for this grip on the forehand, which throws the timing off for many shots. It can make the backhand too wristy as well, also making it difficult to learn to control the ball.
A backhand grip may make forehand looping awkward as it tends to tighten the arm up on forehand shots as well as making it more difficult to close the racket against an incoming topspin, or to loop anywhere except crosscourt. It can force the back shoulder to hunch up in an awkward attempt to adjust for the naturally overly open racket angle for this grip on the forehand, which throws the timing off for many shots. It can also make hitting aggressive backhands awkward since the arm's natural stroke path and the wrist no longer are aiming in the same direction. These are just a few of the problems a bad grip may cause, and there are just as many problems caused by poor foot positioning and playing stance.
At the advanced levels some players do adjust their grips, taking on usually slight forehand or backhand grips. (I generally use a slight forehand grip, but only after I'd been playing ten years.) There are some technical advantages to this, but only after you have ingrained proper stroking technique.
Similarly, make sure you are in at least a slight forehand stance when hitting forehands (i.e. right foot slightly back for righties, making it easier to rotate your shoulders back as you backswing), and a neutral stance when hitting backhands (feet roughly parallel to the table). Advanced players sometimes adjust their stance based on their playing style, and may play forehands from a nearly neutral stance or backhands from a forehand or backhand stance, but again, I recommend against this until you have ingrained proper stroking technique. For example, if you keep your feet parallel when hitting forehands when developing your strokes you'll likely end up with a short, jerky forehand (whether hitting or looping) that uses only the front part of your forehand hitting zone.
Also make sure the feet aren't too close together as this leads to balance problems when looping or hitting with power. The feet should also be at least slightly angled away from each other, with the front of the right foot angled to the right, the front of the left to the left. If the feet are parallel, then it will be difficult to make quick body rotations when you backswing, especially on the forehand side, as well as balance problems on power shots.
Even at the advanced levels players often have trouble with a specific stroke because of their grip or stance. Because they've played this way so long they don't even recognize the cause of their problem, and most often they are destined to an eternity of stroking like a crinkled piece of paper. In a few cases they realize what the cause is, and fix the problem, which often simply means going back to a more neutral grip or adjusting the foot positioning.
Like a piece of paper, if you get the top and bottom parts right, the rest falls into place.