If action is always equal to reaction, explain how a horse can pull a cart ?

Explanation

The force on the cart determines whether the car will move or not. The force exerted by the cart on the horse affects only the horse. Hence, if the horse is able to enough force to overcome the frictional force, the cart will move. Thus, in order to make the cart move, the horse bends forward and pushes the ground with its feet. When the forward reaction to the backward push of the horse is greater than the opposing  frictional force of the wheels, the cart moves.