Address and body placement
This is something that amateurs should be able to do as well as their professional counterparts, but it rarely works out that way. The professional has their left shoulder facing the target with their left foot directly underneath the shoulder. They play the ball midway between their feet. An amateur is likely to open up the left shoulder and not address the ball squarely. Amatuers are also likely to play the ball about a ball length or two closer to their front foot than their back foot.
The professional golfer begins their swing by turning their hips to the right. As the hips turn, the arms and hands follow. The hips are clearly the engine of the swing and get it started. The amateur golfer gets into the swing with their hands and arms. They may know about turning the hips, but when involved in a close match, amateurs will try to overpower the ball by making a fast backswing with their hands and arms.
When a professional golfer begins the downswing, they use their full left side (front side for a right-handed golfer) to begin their pass at the ball. Again, the hips are the engine of the swing. The hips have started to turn to the left and then the left side begins to attack the ball. This is done in a smooth and rhythmic basis. When an amateur swings, they will often begin with their left side diving into the ball but instead of a rhythmic swing it will be done with a swaying action that will prevent them from completing the swing. A swaying motion of the hips often leads to an incomplete swing
One of the most important factors during the swing is keeping your left arm straight and your left elbow locked as you make contact with the ball. The professional will do this as a matter of course because they knows that if the elbow bends they will lose accuracy and distance. However, an amateur golfer may lose control of the elbow and allow it to fly open on contact, usually resulting in a ball that hooks to the left.
A professional golfer knows that finishing the swing is a must if you are going to play well and shoot a low score. The professional will always finish a full swing with their hands and arms at shoulder level. Many amateurs do not finish their swings and pick their heads up shortly after contact because they want to admire their shots. This is a big mistake that can cost 40 yards or more in distance. Professionals keep their heads down through the shot and finish with their hands up high. Amateurs don't do this consistently.