If there is a Person sitting in a vehicle which is moving, how could he find out whether it's in motion and if he figures that out, how does he find out whether it's accelerated or non-accelerated?
If the vehicule is moving at constant speed and the individual can't see the exterior of the vehicule, then there is absolutely no way for him to tell if it's moving.
It's actually a fundamental building block of special and general relativity. All inertial frames are equivalent, meaning that you can't determine an absolute velocity for an object, only velocities relative to you.
If the vehicule is accelerating, then you will feel it, because humans have built-in accelerometer in there bodies. A more fundamental true is that giving differents observers, all of the observers will always agree on which ones are accelerating. There is no ambiguity, in contrast with uniform motion at constant velocity where it is not possible to determine an absolute velocity.