Question

pls answer pls...................

Answer 1

it is called law of inertia, when the bus stops your upper body is not in the direct contact of bus hence when it is stops suddenly your upper body still inherent the motion that is why your move forward but your legs will not move forward because they are  direct contact with the bus. 
and this is also happens when the bus accelerates your upper body move backwards due to law of inertia

Answer 2

We tend to fall forward when the bus suddenly stops because initially the bus was in motion and our body was also in motion, but when the bus comes to rest, our feet also comes to rest but our rest of the upper body opposes this due to inertia of motion.
We tend to fall backward when the bus suddenly starts because when the bus was at rest, our body was also at rest but when the bus suddenly comes to motion, our feet also comes to motion but our rest of the upper body opposes this suuden motion due to inertia of rest.