If an object is motionless, it will stay motionless unless acted upon by some force.
If an object is moving at a constant speed or velocity, it will continue at that speed unless acted upon by some force along the line of motion.
If an object is moving, it will move in a straight line unless acted upon at an angle by some force.
The Law of Inertia assumes there is no friction or other resistive force that can slow down an object. Inertia can be best demonstrated in outer space.