GRAMMAR, PLAINLY.
PREPOSITIONS → IN / ON / AT — PLACE · RULE 10

in / on / at — place

in — inside a space. on — touching a surface. at — at a point: an address, a spot, an event.

FORMULA
in → on → at
ININSIDE A SPACE
in the kitchen · in a box · in Milan · in Italy
ONON A SURFACE
on the table · on the wall · on the second floor
ATAT A POINT
at the door · at the bus stop · at work · at a party
THE THREE PICTURES
in
The keys are in my bag.
inside
on
The keys are on the table.
a surface
at
I'm at the door.
a point
Transport: on the bus, on a train, on a plane — but in a car, in a taxi.
CITIES & DAILY PLACES
cities & countriesin Milan · in Italy
daily pointsat home · at work · at school
floors & directionson the second floor · on the left
TRICKY ONES
in bed
in the sky
on the internet
on TV
at a party
at the top
COMMON MISTAKES
I'm in home.→ I'm at home.home = a point
She is at the kitchen.→ She is in the kitchen.rooms = inside
It's in the second floor.→ It's on the second floor.floors → on