ON - to express the surface of something: (e.g. "The book is on the desk.") - to specify days/dates: (e.g. "Let's meet on Wednesday." / "Let's meet on 23rd of April.") - to express someone using a device, or something in a device: (e.g. "She's on the phone right now." / "My favourite song is playing on the radio!") - to express a part of the body: (e.g. "He slapped me on my cheek.") - to express the state of something: (e.g. "The stove is on fire!" / "These clothes are on sale right now.") AT - To specify time: (e.g. "Let's meet at 4pm later.") - To indicate a place: (e.g. "He's at the hospital.") - To express an activity: (e.g. "He threw a ball at me." / "I'm not good at playing chess.") IN - To express unspecific time: (e.g. "He drinks coffee in the morning." / "The order will arrive in 2 weeks.") - To indicate a specific place or location: (e.g. "He's in the kitchen.") - To express a characteristic (shape/size/colour): (e.g. "We gathered in a circle." / "Does this shirt come in other sizes?" / "Does this shirt come in black?") - To express while doing something: (e.g. "In preparing for the exams, I stayed up all night to study.") - To express an opinion/interest/belief/feeling: (e.g. "I believe in you. You can make it!" / "I'm not interested in religion." From my answer to the same question by another user: http://www.italki.com/entry/534804 Hope this helps! ^_^