Le preposizioni imprevedibili    


  io sono ....
a casa at home,
in casa in the house
in chiesa at church
al bar at the bar
al ristorante at the restaurant
allo stadio at the stadium
al lavoro at work
in biblioteca at the library
in ospedale in a hospital
in ufficio at the office
in classe in class
dal dottore at the doctor's office
a teatro at a theather
al cinema at the movie theater

It is not necessary to memorize them.
Just be aware of the fact that there are many possibilities (not to mention variations)