Gwen is right. Here's a detailed explanation: AT is used for an EXACT TIME (Let's meet at 3pm.) or a POINT, A SPECIFIC PLACE OR A POSITION (She's at the bus stop.) It can also be used for SOMEONE'S HOUSE (Charlie was at Megan's house when his mother called.) You may also use it for EVENTS (Tom was at Mike's birthday party) IN is used for: 1. YEARS, MONTHS, SEASONS or any LONG PERIODS. - I think we met in 2010 - Linda is going to Canada in June. - We love to go to the beach in summer. - There are no mobile phones in 1800's. 2. AN ENCLOSURE OR SOMEWHERE YOU ARE SURROUNDED - I'm in a taxi. - Your bag is in the back of the car 3. A SPECIFIC PLACE (city, town, country) - Are you in Japan? - I live in Asturias. ON is used for a specific day and dates - Can I see you on Friday? - She will start working on 12 August. YOU MAY ALSO USE ON FOR HOLIDAYS - There's a party at Jake's house on New Year's Eve. ON is also used for surfaces: -His house is on the top floor. - Your pen is on the desk. Or for DIRECTIONS - There is a church on the left. That's it.

Whenever you want! The great thing about English is that you are not restricted; you are free to use the language however you please... as long as the other side can understand you; language is communication and must include understanding. The words you're asking about are among the small common words which are in the book "keys to words". Each word has exact meanings and uses. Check it out: http://www.keys2words.info/ My Skype: brian.madden4

prepositions are used in English as they are in German. Can you elaborate on exactly what you are looking for for an answer?

I am not an English teacher but....my native language is english. at is used: "I am at the store" or "Were you at the movies?" or Lets go at 7:00pm on is used: "I am on top" or "Is the show on?" or "Lets do that on Friday" in is used: "I am in the store" or "In the winter I like to go skiing" to is used: "Lets go to the store" or "Are you going to the party?) Does that make any sense to you?

It's worth to mention both TIME and PLACE.