Confused Prepositions

In and At

Rule: in is used to describe the physical location of something; at is used when we refer to an address, place, building, etc. It is also used in cases where the location is not an issue; rather, what we do there is important (school, park, mall, hospital, etc.).

Incorrect Usage: (a) Henry owns a mansion at New York.

  (b) The British Prime Minister stays in 10 Downing Street.                                          

Correct Usage: (a) Henry owns a mansion in New York.

                             (b) The British Prime Minister stays at 10 Downing Street.