In a landscape painting your light source should be clear. It may sound a little basic but if your light source is confused 9/10 your painting will be confused. When considering landscape painting it is really the sun that is your main source of light. Shadows are the result of this with their density and sharpness being determined by the brightness and time of day. A strong light source will give your painting a great range of tonal values, strong shadows and helpful indication of the time of day.
The painting above would be flat and uninteresting if it wasn’t for a clear and interesting light source and the shadows it creates.
Above is another example of there being a clear light source so well defined shadows can be seen.
Top tip Show on the edge of your paintings with an arrow, in pencil, the direction of your light-source; it is a great reminder while you work!