At 1, at sb's (at sb's house) 2, at home 3, at work (in the place where sb works) 4, at the doctor's/dentist's/hairdresser's 5, at a party/concert/meeting In & At arrive in Japan (a country) arrive at Tokyo (a city) In & At at school (British English) in school (American English) "I work as a secretary for ??? company". (Correct) "I work as a secretary at our company". (British/Correct) "I work as a secretary in our company". (American/Correct) But, "I work as a secretary [on] our company". (Wrong, Incorrect)

As I know those prepositions can be referred to some indications.they are place and time. First, preposition of location /place. At : this preposition indicates that noun follows it is treated as a point in relation to which another object is positioned. for examples : 1. I am waiting for my girlfriend at the bus stop. 2. They arrived at the house. On: this preposition indicates the location of noun is defined with respect to surface on which it rests. for examples: 1. They're playing football on the field.(not enclosed) 2. The cat is on the table. In : this preposition indicates that noun is confined in an area. for examples: 1. I live in that building. 2. The two boxers are in the ring (enclosed by ropes) Second, preposition of time. at: He comes to class at 7.00 am. We usually have lunch at noon. in: He studies English in the afternoon. We usually have coffee in the morning. on: We'll meet John on Sunday morning. I will have my vacation on this Monday.

i have the same question as you. so i asked another question about 'at,on,in' http://new.italki.com/answers/question/36125-1-1-1.htm