Shadows give object weight and dimension. Dark shadows under plants, trees and rocks help to ground them. The length and direction of a shadow depends on the direction and angle of the light. Everything casts a shadow, so your painting becomes real when they are included.
Dimension can be produced with the use of dark and light colors. Dark moves back in a painting. The contrast of painting light objects over a dark color brings the lighter objects forward.
A light color painted on a white canvas will look washed out. First paint a dark color in the desired shape. Let the canvas dry. Then paint the light colors over the dark. The contrast will help the light color pop out instead of disappearing on the canvas.
Use of a ruler: bottom of the ruler or yard stick is against the canvas. The top of the ruler is held against the canvas and the bottom is held away. The hosle of the brush is held against it the ruler or yard stick brush straight along the ruler.