I am at home. I am in bed. I am at school. I am in the classroom. It is in the wardrobe. He cannot sleep at night. She looked at you like you were to blame. They were at war in 1986. Definitions. At: expressing location or arrival in a particular place or position, expressing the time when an event takes place. In: expressing the situation of something that is or appears to be enclosed or surrounded by something else, expressing a period of time during which an event takes place or a situation remains the case. I hope this explains it for you.

Usually you can work it out by thinking: Is A completely surrounded by B？ If so, A is in B. But there are lots of exceptions too. So: In a city. At the corner. At the shops. In the shop. Park can be either.