if you wanna do some awesome modelling (or 'rendering')...
get a reeeaaaaally nice, sharp, pencil. the sharper your pencil, the sharper your awareness.
do a nice ol drawing and get it accurate. get your shadow shapes where you want them, whatever.
now, do some hatching. look down at your paper... notice how inbetween your pencil marks theres those white spaces? fill those in (with your sharp pencil!). use a kneaded eraser to pick out those little dark spots. This flattens out your shadow shape, which is what you want for now.
Then, model from the shadow up to the light. Make sure you fill inbetween the marks if you want that smooth look.
So you'll have the full value range between your shadow shape and the white of the paper to work with. later on, when youve established your value range, you can pull out your reflected light in the shadows.
Of course, what I just told you is a process, not the actual ideas behind the process. But I cant teach you that in a forum. So, if you follow what I said, you'll learn to make some nice pretty drawings; even if theyre not totally right.