Forms are three dimensional shapes. Because drawings and paintings have only two dimensions, artists must use shadows on the objects themselves as well as cast shadows in order to let the viewer know if the object they are viewing is a form, or a flat shape. If you learn how to create form, you can make everything in your artwork look 3D. Most objects that we have in everyday life are made up of combinations of slightly modified platonic forms.
Malia, age 10
Sam, age 14
Monty, age 10