Understanding how hard or soft the cue ball needs to be hit is just as important as knowing how english will effect the ricochet off of the object ball.
Note: To make sure that you are getting the correct speed effects, make sure to test out the same shot 5 or 6 times exactly the same way with the same speed and see how the results differ.
The harder you hit a shot, the more accurate you must be in aiming. Being slightly off could cause the object ball to juggle in the pocket and bounce out rather than fall down. On the other side, hitting too slowly could bring dirt and felt tears against your shot going in.