How did World War I change the way in which wars are fought?

Most dramatically, WWI changed the nature of warfare from armies marching in an open field and facing each other in formations, commonly known as "maneuver warfare", to trench warfare. It was also the beginning of Mechanized Warfare, seeing the introduction of tanks, gas, cars, the Maxim machine gun, and other new weapons.

You ask a very complex question... this is just the gist of the answer.