Why does a passenger keep travelling forwards if they are not wearing a seat belt in a crash?

A passenger continues to move forward during a crash due to inertia, which is the tendency of an object to resist changes in its state of motion. When a vehicle suddenly decelerates or stops, the passenger's body, not being secured by a seat belt, continues at the same speed the vehicle was traveling until another force acts upon it, such as hitting the dashboard or being restrained by another object. This is a demonstration of Newton's first law of motion, which states that an object in motion stays in motion unless acted upon by an external force.