If you push a heavy box at rest, you must exert some force before it begins to move. Once its moving, you don’t have to as much force. Which of Newton’s three laws explains this?

Under the action of a constant force an object accelerates at 7.8 m/s2. what will the acceleration be if (a) the force is halved? (b) the object's mass is halved? (c) the force and the object's mass are both halved? (d) the force is halved and the object's mass is doubled?