The prepositions "in" and "at" are used differently in American and British English. Other prepositions are used differently in our two dialects as well. See the following Wikipedia article: http://en.wikipedia.org/wiki/Comparison_of_American_and_British_English From the article (my examples are for American English): 1. British athletes play in a team; American athletes play on a team. (Both may play for a particular team.) *** He plays for the New England Patriots. *** Who plays on the New England Patriots? 2. In American English, one always speaks of the street ON which an address is located, whereas in British English In can also be used in some contexts. *** I live on South Maple Street. *** I live in a big house. *** I work at / in / for a bank. 3. British English favours the preposition AT with weekend ("at (the) weekend(s)"); the constructions on, over and during (the) weekend(s) are found in both varieties but are all more common in American English than British English. ** We never say "at the weekend" in America. We say "on/over/during the weekend". *** What did you do over the weekend? *** Did you do anything fun over the weekend? *** On weekends, I like to go to the beach. Some examples of "at" in American English: 1. Mark: Where is Adam? -- Steve: He is at the beach. 2. Mark: Where is Adam? -- Steve: He is at Walmart. He wanted to pick up some bottled water. Some examples of "in" in American English: 1. He likes speaking in Irish. 2. There is a dog in the kennel. 3. They are dancing in the street. (I think British English actually uses "on" here instead. These are actually the lyrics to a famous American song though. :) ) 4. There is a woman in the crosswalk blocking traffic. 5. He is in trouble. 6. Can you put this in the microwave for 5 minutes? 7. In the fall, I like to carve jack-o-lanterns. 8. In an emergency, please call 911. 9. He is in a bad mood. 10. There was a fight at / in the mall.

There are no general rules.