Before I continue, let's distinguish form from shape. Per artist and instructor Marshall Vandruff, the simple difference is: a shape is flat; a form is thick. Here's how Andrew Loomis illustrates the difference in his "Fun With a Pencil" book:
Pages from Glenn Vilppu's "The Drawing Manual":
A page from the original Famous Artists Course:
Screenshots from Stan Prokopenko's free Drawing Basics series:
From the Watts Atelier drawing course:
Clearly, the understanding of forms should NOT be overlooked. I'll be spending a lot of time drawing these basic forms and training my eye to reduce everything I see into those forms.