There are many ways to create the illusion of space and depth in a picture. One of the simplest is by placing two objects next to each other on the picture plane, one small, and one big:
The Stars My Destination, by Donato Giancola
We know the earth is bigger than one man's head, but pictorially we see the earth as small. This means it must be very far away. You can see the same effect with these boats:
Gloucester Boys, by Winslow HomerThe children in front are large, and the ships are small, so they must be very far away. This watercolour also shows overlapping. The heads on the left cover some of the ships, so the boys must be in front.
Here's another example of overlapping. Look at all the people and things on the right and left. They're organized almost like dominoes, one in front of the other, to create a strong illusion of depth. The more "dominoes" you can add in a line, the farther back in space they will appear to go.
Kongens Nytorv, Copenhagen, by Paul-Gustave Fischer
Also note how these receding shapes on either side help frame the girl in the middle.
Atmospheric perspective is when landscape artists mimic the effects of air as objects get farther away. The colour and value of an object changes as you walk farther and farther from it, because the air itself has a colour - usually blue. Distant objects are usually lighter, duller, and bluer than objects up close:
Antignano Outskirts, by Gambogi Danielson
Whether the sky appears blue depends on the position of the sun. Around sunrise and sunset, the sun is positioned at a low angle, and turns the sky a variety of colours - red, pink, yellow, etc. It also depends on the humidity and clouds.
Lake Nemi, by Samuel Gifford
There's nothing blue about that distant light.
Another effective strategy to achieving a sense of depth is to organize your shapes according to height placement. Whatever is at the bottom is closest, and whatever's at the top is farthest away. This approach is typical of Asian arts:
Landscape in the style of Ni Zan, by Hong Ren
Linear perspective is the most accurate way to represent the illusion of space in a picture. It's also the most complicated. There are different kinds of linear perspective: 1-point, 2-point, and 3-point. They all use a horizon line with vanishing points on it, with everything in the picture receding to those points. In reality, there are six points of perspective: front & back, left & right, top & bottom. But in most works of art, you only need one to three.
Linear perspective works best with architecture and geometrical shapes. But, it also exists in nature.
Trees & Sky, by Kari Liimatainen
Clouds, by Akseli Gallen-Kallela
Everything you see is governed by the rules of linear perspective, including organic and irregular shapes - even you.
1-Point perspective is the simplest. This is ideal for hallways and city streets:
2-Point perspective is best for viewing buildings at an angle.
3-Point perspective is necessary when looking up at very tall buildings.