"On the bus" is
a fixed conventional expression in all varieties of English across America and
Britain. "On the bus" is used in the transportational context (i.e., you are on the bus when you are travelling by bus) while "in the bus"
can be used to show your position (in the bus). A similar example is "on
the train". You say "on the train" when travelling by train
whereas you use "in the train" to indicate your position in relation
to the train.
However, the
reverse is the case when it comes to car. You are "in the car" when
travelling by car. In this case, it is inappropriate to use "on the
car" except you are on top of the car. It is also a fixed conventional
expression.
You can use
the hints below to get them right:
1. You get OUT
of a car, so you get IN it.
2. You get OFF
a train, so you get ON it.
3. You get OFF
a bus, so you get ON it.
IN for OUT,
and ON for OFF. It is just like using the opposite of one to get the
other right.