There is no one single rule that will help you use "at" or "in" correctly every time. The difference depends on the context and there are many, many rules and fixed expressions you have to memorize. Here are a few: 1. AT + street address (I live AT 123 Main Street.) IN + city, state/province, country, region, continent (I live IN Canada.) 2. AT home, AT work, AT the office 3. I'm AT school. = right now I am inside a school building I'm IN school. = I am a student; this is my primary occupation 4. IN the hospital = I am sick (British English = in hospital) AT the hospital = I am inside a hospital but I am not sick. (Maybe I work there or I am visiting a sick person) 5. IN January/February/March/etc. IN the morning/afternoon/evening AT night There's no special reason why some of these expressions use AT and some use IN. It's just our habit as native English speakers!

In general, AT is used for a especific place. IN is used for general place. Eg.: I'm at Magie's house (Especific place: Magie's house) Eg.: I'm in The USA (which place in The USA?, you don't know)

Ok, so "at", is used to say you are in a certain place. "I am at the movie theater", meaning you are physically located near or around it. "In" is generally used to say you were a part of or located inside something more general. If I were to say, "I am in the movie theater", I would be saying I am inside the location I am on the premises of it. You can also use "in" as saying you were part of something, such as, "I was in a play", meaning you participated or took place performing the play.

At, to be or to go to a place. In, to be or to go inside a place.