A passenger is standing in a train. The train accelerates and the passenger falls backwards. Use Newton's first law of motion to explain why he fell backwards.

First we want to state what Newton's first law is. 

Newton's first law of motion states that an object at rest will remain at rest, or if in motion, will remain in motion at a constant velocity unless acted on by a net external force

The friction between the floor and the passenger’s feet creates a net force on the feet. Then when the train accelerates the passenger continues travelling for a small instant at the original velocity. So the feet move forwards faster than the rest of the body causing the person to fall backwards. If you are still confused draw a simple diagram with a stick man and add arrows for the direction of travel and direction of acceleration.