This is something I need to burn into my artistic brain forever. Drawing with the form principle in mind can help us render things more easily and successfully.
Here in Successful Drawing by Andrew Loomis we can see examples of the form principle.
Basically, when a light strikes a geometric solid, like a sphere or a cube, it creates an orderly series of tones. From highlight to halftone to form shadow to cast shadow. And when we learn to identify these tones and place them in their proper relationship, we will then be able to create solid looking drawings and paintings with believable light and shade.
Look around and you will start to see geometric shapes everywhere! And once you discover what shape something is, you simplify it and then you can better assess how light is affecting that shape.
Here are some great things to remember from Loomis: Excellent rendering of light and shade produces design and the design is what makes a picture appealing, not the subject matter!