Form and Cast Shadows

I am embarrassed to admit I had to have form shadows on a human body explained to me. It just wasn't registering. I was in class today painting a nude model. I was told to count my brush stokes. I had 170 before blocking in everything.
I have to learn longer strokes.
Then, I was told to distinguish between form and cast shadows. Now, I know a cast shadow is sharp but as for putting it down with oil on a curvy body. I was clueless.
After an excellent demo, I understand.