How do we know when we are supposed to add the velocities and when to subtract them?

add the velocities when they are in opp. direction and subtract them in same dircn.

plz approve!

when two bodeis are moving in front of each other then we add their velocity 

but if they are moving side by side then we subtract their velocities

proceed by vector addition. this is the best method.

only when they are in the same line that is collinear vectors then you may think of adding or subtracting their magnitudes.

if you are really confused then think of a simple situation.

imagine that you are travelling in a car or in a train.

when another vehicle crosses you in a direction opposite to yours then you feel that the vehicle appears to be coming towards you with a speed more than with what it is actuallly travelling that is it appears to be more fast simply because you are also moving towards it.

in the other situation when you are following something then that appears to be slower than it actually is.

so now you are clear when to add and when to subtract.

but be sure that the velocities are in the same line first only then this method is applicable.