When you draw, you use lines. When you paint, you use light and shadow. To paint realistic, 3D looking forms, you need to understand how various shades create the illusion of depth. 

The basic principles are easy, you can probably tell the difference between light, shadow, and reflected light. But the real question is: how do you predict the shape of the shadow? How do you guess whether the surface should be bright, dark, or somewhere in between? To create convincing shading from imagination, you must be able to answer these questions. In this course, you'll learn what rules you can use to predict shading accurately, and how to simplify them to shade animals quickly and intuitively.

Home Page - https://design.tutsplus.com/courses/how-to-shade-animals-in-perspective