PREPOSITION IN, ON, AND AT

 The prepositions in, on, and at can be used to indicate time and place. Notice how they are used in the following situations:

Preposition	Time	Place
In	Year, Month, In 1999, In December	Country, State, City In Japan, In Utah, InTaipei
On	Day, Date On Saturday, On May 1	Street On Main Street, On 1st Ave.
At	Time At 8:00, At 7:30	Address At 815 East Main Street

In many languages, there is only one preposition for the above situations. In English there are three. Just remember that in usually indicates the "largest" time or place, and at usually indicates the "smallest" time or place. Examples:

A: Where's your office? B: In Taipei, Taiwan. A: Really? What part of Taipei? B: It's on Chung Shan North Road. A: I know that area. Where exactly is it? B: It's at 105 Chung Shan North Road, next to the bookstore. C: When is the wedding? D: It's in June. C: What day? D: It's on Saturday, the 25th. C: What time? D: It starts at 6:00.

Prepositions with articles and locations

When talking about locations, use at to indicate the general vicinity or area, and in to
indicate inside the building, enclosed area, etc. For example:

at the swimming pool (on site)	in the swimming pool (in the pool itself i.e. in the water)
at the post office/bank (general)	in the post office/bank (inside the building)
at the zoo (visitors, general area)	in the zoo (animals in their cages)
at school	in the classroom

Sample sentences:

I met my wife at the theater. (while watching a movie) I spilled my drink in the theater (on the floor of the building) She works at the library on Wednesdays. She found a rare coin in the library (building). Dr. Jones works at the hospital every day. John was in the hospital for a week with a broken leg.

For school, prison, and church, the is used to indicate the building. No article indicates
the general situation. Note the following:

"practice"/situation	building
in school (studying, listening to teacher, etc.)	in the school (building)
in jail/prison (staying there as a criminal)	in the jail/prison (temporary)
in church (praying, listening to a sermon, etc.)	in the church (building)

Where's Dad?

in mosque (attending services)	in the mosque (fixing the windows)
at mosque	at the mosque
in prison (He committed a crime.) Source : http://www.eslgold.com/grammar/in_on_at.html	at the prison (visiting his friend)