The way light moves over people or objects shows us their shape and form. In landscapes, at least dayscapes, the sun is the light source. Look closely outside, in natural light, to see how the sunlight is affecting your subject. The seasons and weather conditions will have an affect on the color and character of the light.
Determine where the sun is in relation to your subject. Is it overhead? Think Hopper, with harsh glaring light and strong contrasts. Shadows will be reduced in size, but not strength. The lightest and warmest areas will be the flat planes of the landscape. Sometimes on an overhead, overcast day there’s a wonderful radiating light.
If the sun is striking the landscape from the side, strong contrasts will reveal the forms. More texture will be visible and long shadows can add a design element to your composition. Colors deepen and are cooler going away from the light source.
Sometimes the subjects are backlit. The sky will always be the lightest and warmest part of the painting in backlit paintings and the forms will be in shadow. Tones are deepened where the light strikes. The backlighting can be unifying and set a mood for the painting.
When choosing and using the light for your landscape the most important thing to remember is that the sunlight must come from a single, consistent source. Don’t have your shadows going in different directions!