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preposition position

The preposition (At) we use to say when we are at a location, event.
Example: at the party, at school, at the bus stop, at the table door window etc.

We use the preposition (In) when we mean the building container house etc.
Example: in the building, in the school, in the church, in the hospital,
in bed. However on the bed.

Sometimes we can use either in or at.
In school or at school
in college or at college
in the movie theater or at the movie theater etc.

We use in with city names and country names
In Holland
In America
In Taiwan
In Taipei
In Tokyo
In Hon-Kong

I hope it helped a little.