I want to know the reason

[Unregistered]

But our english teacher said that there is a reson for why we use ''by THE bike'' and by bus

[Barb_D]

I'm not quite sure what your asking, but it's commonly said:

I go to school by bus.
I go to school by car.
I go to school by bike.
I go to school by foot (or on foot - that one goes both ways).

We would not say "I go to school by the bike." Are you quite sure that's what your teacher said is correct?

[Raymott]

But our english teacher said that there is a reson for why we use ''by THE bike'' and by bus

Who's "we".
If I'm going anywhere by bike, I'd say "I'm going by bike". "By the bike" sounds odd.
If I'm going into the city, I might take the bus, ie. go by bus.