Fundamental tackling is something the D needs to review. You can't hit hard without knowing how to position your body and the aiming points of the runner/wr you are going to tackle.
Get low. Proper tackling form involves getting low and keeping a roughly 45 degree angle with your back, which should be straight because your shoulders are back. Squat with your feet still shoulder width apart and bring your hands up into what is sometimes called the "Breakdown" position, your basic tackling form.
1. you are looking for the near foot and near shoulder of the person you are tackling
2. Approach the ball carrier, chop feet, aiming point is the inside hip and lower trunk. Engage ball carrier with shoulder pads (same foot, same shoulder), and head up. Tackle through the ball carrier, do not stop feet. Grab cloth and finish the tackle keeping your head up
3. Tackle/ explode through the body feet constantly moving which will lift the ball carrier of his feet and take him to the ground