The prepositions at, in and on are used in several fixed expressions.
In means that you’re really inside a place, surrounded by a three dimensional space. Take a look:
In a room (numa sala)
In a building (num prédio)
In a box (numa caixa)
In a garden (num jardim)
In New York (em Nova Iorque)
In Brazil (no Brazil)
At has a more general meaning of location.
At the bus stop (no ponto de ônibus)
At the traffic lights (no semáforo)
At the door (à porta)
At the window (na janela)
At the end of the street (no final da rua)
At the top/bottom of the page (na parte superior/inferior da página)
At reception of a hotel (na recepção de um hotel)
On shows the idea of contact with a surface.
On the ceiling (no teto)
On the table (na mesa)
On the floor (no chão)
On the wall (na parede)
On the ground (no chão)
On the grass (na grama)
On page 7 (na página 7)
On the bulletin board (no quadro de avisos)
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