When do single forces happen?
It depends on what you mean by a "single force" and "happen". In physics, which is the branch of science that explains the physical goings-on around us, you might possible be referring to the concept of "unbalanced forces". Unbalanced forces are when you are exerting a force in one direction while another force is being applied in another direction, but is larger in magnitude than the first force. A good example of this is two teams playing "tug of war". The team that exerts the larger force on the rope and maintains good traction with their feet, will pull the other team towards them. What "happens" is motion, pure and simple, in a given direction with a given net force.
If forces are balanced, on the other hand, the net result is zero and no motion is produced. A book resting on a student's desk is pushed up with a force that is equal to the downward force of attraction the Earth exerts on the mass in the book. What "happens" here is no motion: the two forces are balanced, or equal.