Explain in terms of Newton's first law why you should never step off a bus or train before

it has completely stopped.

Newton's 1st law demonstrates that unbalanced forces impact the motion of yourself when you get off the train. You would most likely lose your balance due to the difference in motion between the train and the ground.

Explanation:

Newton's first law states:

An object in motion stays in motion and an object at rest stays at rest. However, this changes if the object is acted upon by an unbalanced force I. E. You are on the train, meaning that you are in motion with the train and are moving at the same velocity. If you get of the train while its moving, you are still moving at the speed of thr train. Therefore, your momentum is forced to stop when you hit the other ground; causing you to lose your balance.