Form is seen due to the effects of light, and the way in which that light reflects, is absorbed and casts shadows. The values and placement of these shadows and highlights can tell us many things about the object's softness, coarseness and reflectiveness to its sense of density and depth… A cloud has form as does a rock, both have edges and are defined by light shining upon them and yet if you drew a cloud as you would a rock it would look too heavy and dense to float convincingly.
Every time you come to draw an element from a scene, visualise that element in your mind – mentally run your hands over it, is it coarse, sharp, furry, rigid, solid, spacious? Note the direction in which your light source(s) comes from, and note again how your object responds. All of these things will determine how you come to draw the object before you. A cloud may require light shading and blending of edges; a rock may be much darker in contrast and require more pressure from a pencil to create its harder edges, and note that some rocks have sharper edges than others, just as some clouds can have sharper edges than other clouds - a storm cloud will be darker and angrier than a singular white cloud on a midsummer's day. A reflective surface will reflect more light, a matt surface will absorb, a spacious object like a tree will reveal the light of the sky between its branches and leaves to suggest its many limbs (it is common for a beginner to mistakenly render vegetation as a solid lump).
Such observations all accumulate towards the art of seeing, and as art is primarily about seeing the world, the next time you are out walking, try visualising the things you see in terms of their form, taking into account the sun’s position in the sky.
Basic Geometric Forms
Try these simple shapes without the use of a ruler. Draw a square, rectangle, circle, triangle and ellipse. Most of these shapes can be used in a drawing (see the Giraffe sketching demo), and tailored a little to fit the subject.
Asides from the sphere and ellipse, these shapes can be drawn by using receding parallel lines, roughly at a 45 degree angle. If you find it easier, look for examples of these forms in your house, and try sketching them - a TV, flower pot, book, cup...
If you can find some common household objects that hold a simple geometry, place them up to a singular light source, and observe how one side is cast into shadow, whilst one catches the full light, and the other side lies somewhere between the two (see photo at top of page). On curved objects like a cylinder, the reflection will catch part of the object and graduate into darkness as the object curves away into shadow.
A sphere can only be represented by shading as it has no edges. There is an area where the light source reflects the brightest, and this we call the highlight, but it is also interesting to note that even in the area of shadow you will often see some reflected light coming back into the shadow area, and it is important to bare this in mind.
As a quick freehand exercise, try grouping a few 3D objects together into a small assembly.