at + events. For example:

• I was at a party last night.
• Where is David? He is at a concert.

at + building. For example: She’s at the library with her friends.

However, you can use in + building to emphasise that the person is inside the building.

For example, “I’m in front of the library. Have you seen Jane? Where is she?” “Yes, she is in the library."

at + a specific place. For example:

• The postman is at the door.
• There is a supermarket at the end of the street.

at + stop on a journey

• We stopped at a small village.
• The train from Manchester to London stops at Birmingham.