One of the most crucial aspects for getting a landscape painting to look authentic or realistic is to have the direction of the light consistent across all the elements in a painting. Actually, this 'rule' applies to any subject you're painting, unless you're a Surrealist perhaps. When you're still at composition stage, you need to decide which direction the light is going to come from as this influences the shadows, contrasts, and colors. If you're plein-air painting, this means waiting for a particular time of day for the sun to be shining the 'right' way.
It can get more complicated than this if, for instance, there's light reflecting off a surface. But let's stick to the basics.
It's well worthwhile playing around with an angle-poise lamp (if possible, use a daylight bulb) and a simple still-life setup to really get to grasp with light direction and shadows.
Move the lamp to the side, back, front, and into an elevated position. Put a sheet of paper over it to diffuse the light. Sketch the various scenes, taking particular note of where the shadows falls and where the highlights are. Look at the colors and how the different directions of light influence this and the appearance of the objects.
This knowledge will enable you to apply a light source consistently and effectively when painting (and it's still relevant even if you're painting from your imagination). It also helps interpret what you're looking at when you're painting a landscape and to be aware of how the light changes.
Note:The options are described here with application to a landscape painting, but apply equally to any subject.