How does inertia work?
Inertia is resistance to a change in motion. An object at rest remains at rest, and a moving object continues at constant velocity in a straight line unless acted on by an unbalanced force. Inertia also belongs to stationary objects.
Mass and net force
Newton's first law describes this behavior in inertial reference frames. Mass measures translational inertia: the same net force produces less acceleration in a more massive object. Inertia is not an additional driving force. An object sliding on ice slows because of friction and air resistance, not because inertia runs out. Balanced forces do not necessarily mean rest; motion at constant velocity is also possible.
Inertia in everyday life
When a bus brakes, passengers tend to continue forward; a seat belt supplies the force needed to slow them with the vehicle. Speed must be distinguished from acceleration: a vehicle moving rapidly at constant velocity may have nearly zero net force. Earth is not a perfectly inertial frame, although its rotation can be neglected in many small-scale experiments. Precision measurements require more careful frame selection.