Very often, I encounter starting out artists who consider line art drawings and shading/rendering work as two separate things, the second one being more professional, harder, more demanding, etc. I don’t understand that division, since both are just a method of representing three dimensional objects in the 2d space. They require the same knowledge – what exactly?
What you need is the awareness of the 3d forms behind what your drawing – especially, the planes, which compose those forms. It’s easier to think about them in case of angular figures, but what about spherical or cylindrical forms? Exactly the same as the others. As you probably remember from your maths class, a sphere has infinite number of planes. As an example of this, here’s an computer rendered example of an angular shape, progressively expanded into something more spherical, but still not a perfect sphere. It’s easier to think of simple objects first – and simple objects can always be expanded to be more complex or smooth ones.