Form is the three-dimensional quality of an object is it appears on two-dimensional film. Through the gradual shading of highlight to shadow, each indentation, curve, buldge, and edge is captured creating depth and dimension.
To quote noted photographer Boyd Norton, "Shape becomes form when we use light to sculpt them into three-dimensional objects". Conversly, forms can become flat without the proper use of light.
That said, it is obvious that the direction and angle of light upon an object (or scene) will have a profound impact on its apparent depth. Up to a point, longer shadows provide the needed depth cues to percieve the object in all three dimensions. Thus the low angle of the sun at dusk or dawn provides more shadow areas emphasizing form and texture.