Newton's first law of motion

A body stays at rest, or keeps moving with constant velocity in a straight line, unless a net external force acts on it.

Put the other way: zero net external force on a body means zero acceleration; several forces may act, but they must cancel.

A book resting on a table has zero net force on it: its weight is balanced by the normal force from the table. These two are not an action-reaction pair.

Everyday examples of inertia: passengers lurch forward when a bus stops suddenly and backward when it starts suddenly.

The law defines the kind of frame (an inertial frame) in which Newton's laws hold as stated; a frame accelerating relative to such a frame is non-inertial.