In, on, at

Lesson 20

I am in my office.
I see two pictures on the wall.
You can sit at the table.
He works at the airport.

When we speak about something inside something, we use in.
When we speak about something on something, we use on.
When we speak about something next to something, we use at.
When we speak about something inside space which is around something, we also use at. For example, at the airport, at the station, at home, at work, at school.

I am on an island. I sleep in a tent.

You can see more examples in the book Robinson Crusoe.