A ‣ At, on and in
At a point or place, e.g. I met her at the bus stop. He’s at work at the moment.
On a surface, e.g. The book’s on the desk. They sat on the floor. I put the picture on the wall.
In an area, space, or inside something, e.g. He’s in the kitchen. She lives in Warsaw/Poland. The knife’s in the top drawer.
Common mistakes
I met them at the airport. (NOT I met them on the airport.)
There’s a computer on my desk. (NOT There’s a computer in my desk.)
The conference is being held in Delhi. (NOT The conference is being held at Delhi.)
B ‣ Where exactly? 
I know they live in Danvers Street, and I think they’re at number twenty-three.
I’m sure there’s a chemist on the left before the bank. [first there is a chemist, and then a bank; opp after]
Their house is beyond the farm [on the other side of the farm], by [near] the old church.
I saw your bike in the back garden against the wall. [touching the wall]
They’ve just bought a house right [exactly] beside/by [next to] the river.
I found your phone on the sofa beneath/underneath a pile of cushions. [under]
Their office is above the shop (opp below).
You can just see the top of the building among the trees. [somewhere in the middle of the trees]
C ‣ Movement 