An object in motion will stay in motion, while an object resting will stay resting. Inertia takes place here because if there is no force acting, an object will not move.

Newton's second Law of motion is that acceleration of an object is based off of the force and mass. The equation for acceleration is acceleration equals force divided by mass, which is a=f/m. If you were to use more force on an object, the object would accelerate faster. If the force stays the same, but the mass of the objects changes, it will decelerate.

For every action there is an opposite reaction. Inertia applies when an action is not applied.