There are three key ingredients that make up a good painting: light & shadow, abstract shapes, and color. Of course, there are about 50 more ingredients that help out, but without these three the results will disappoint.
Another word for light & shadow is "value" and the artist uses a value scale of 1-10 to decide how dark or light a particular color mixture is. Squinting is the happy habit of the painter. Try it yourself: take a moment to look at the room you're in, now squint your eyes until what you see is reduced to only dark and light. This is the beginning of a painting composition! It's a very flat plane of abstract light shapes and shadow shapes. Some of the dark that you see are recognizable objects, like a chair or lamp, but you are also seeing the cast shadow and the form shadow of the object which gets included in the overall dark shape. Now it's not a chair. It's an abstract area of dark. If you sketch that shape out on paper and put in the other dark shapes you see in the room, leaving the light areas alone, the result will be a simplified value sketch, a roadmap to follow as you begin your painting.
Kevin emphasized how important it is to understand that every color (and there are so many!) in the shadow areas must be painted a darker value than anything that is in the light. Sounds simple, but this is where ms. left brain and ms. right brain wanted to mud wrestle in my head. Not only was SEEING and IDENTIFYING the lightest "shadow" crazy difficult, but REMEMBERING that the lightest color value in the shadow still had to be darker than the darkest color value in the light area. Try saying that to yourself 10 times! We 20 brave students gave our best efforts in the mid-80's autumn sunshine of Tennessee and by our third painting we all passed the bar!