One of the most common problems with beginners is they don't rotate their shoulders on the forehand. Several players have this problem in beginning/intermediate class I teach on Monday nights. Even when they learn to rotate the shoulders when hitting forehand to forehand or in multiball they tend to fall back on arm only (i.e. no shoulder rotation) when doing footwork.
The solution I've found is to emphasize the rod-through-the-head coaching technique. When you hit or loop a forehand, imagine a vertical rod going through the top of your head, and rotate around the rod. In reality, the head normally moves a little forward doing the stroke from the back-to-front leg weight transfer, but often very little is needed since most of power comes from torque, as the body rotates in a circle. So for beginners especially it's important for them to focus on this idea of rotating their shoulders around this rod through their head. This gives them the right feel of the shot, and something to focus on to fix the shoulder rotation problem - and when they do footwork drills, it tends to stick with them and they continue to rotate the shoulders properly.
If you watch most world-class players, you'll find that much of the secret to their ability to produce great power and recover almost instantly for the next shot is this idea of rotating in a circle, so they end up balanced and ready for the next shot. The head does move forward or sideways some (and often up), and does so even more when rushed after stepping around the backhand corner to play forehand, but in general most of the movement is circular, creating torque while staying balanced. (Two keys to balance: keep weight between your feet, and use your non-playing arm as a counter-balance to your playing arm.)