Let’s say we have a horizontal spring system with a spring attached to a wall, and a mass attached to the spring. If I pull the mass back, stretching the spring out, and then let go, the mass will accelerate, and the spring will begin to compress. I understand that as common sense.
According to newton's third law, if the spring is pushing on the mass with a force of for example $5N$, then wouldn’t the mass also be pulling on the spring with a force of $5N$? If the spring is exerting $5N$ of force onto the mass, and the spring is being pulled with $5N$ of force by the mass, why would there be any displacement at all?