To help understand the ideas of form and tone when drawing a simple exercise is to tear a strip of paper and make it into a curl or scroll. Set it onto a plain white piece of paper or any plain surface and make sure there is a light source shining onto it so that good shadows are created.
Draw this quickly three or four times for a minute on each. Work towards seeing the shape adn where the lines intersect. Once you have this orgainised in your head start taking a bit more notice, take two or three minutes for each drawing and draw the paper again begin to look at the colours you can see in the white paper, you will be suprised how many greys there are and the depth of tones which make the shadows.
Remember, as you start to add shade to a longer drawing, to consider the whiteness of the highlights on the paper, perhaps along the top edges or on the curve nearest you (depending on the where you have placed the light source).
Move the paper two or three times and do the two minute drawings over again. Study the shape and tone.
Finally take a longer ten minute drawing and try to bring everything you have observed into the final drawing. As you draw take the time to see just how colourful you paper curl is. It will be reflecting the colours in the room, or of the nearby table top…or even your coffee cup. Whilst you are drawing in a monotone its good to be aware of teh colours you can see.