Form is when a light is coming from a upper right corner when this happened it is called a light source in this the start is the light source. The mid tone is on the right of the cube that is a little darker but not to dark or not to light. The top is where it has the most light called the highlight it is the lightest thing on the cube. The darkest part is called a core shadow it is where there is no light at all. The way that I created this illusion of form is I put the start as the light source made a cube with the pen tool and used a different color for each one and made the front the darkest the right side mid tone and the top the lightest.