1) Round forms show gradual shading from light to dark,
and reflected light at the far side of the object, furthest away from the light source - this is vital for giving the impression of roundness. e.g in round fruit and vegetables, heads...
2) Cylindrical forms similarly show gradual shading and reflected light, giving the impression of roundness. e.g in arms, legs, branches, tree trunks...
3) Conical forms similarly show gradual shading and reflected light, giving the impression of roundness, e.g in conifers, skirts, lampshades, an animal's body, anything conical...
4) Square forms show sudden, sharp division between light and dark, and there will be some reflected light in the side furthest away from the light source. Examples - buildings, boxes, books...
5) Note the shape of the shadow cast by the object in each case - it will be a distorted version of the shape of the object itself, and will point directly away from the light source.