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! ^_^

The best way to learn about the prepositions is to find a good grammar book or website. This forum is not a good place to explain all of the rules of prepositions because most of us are not grammar experts, and we might mislead you or leave something out. There are many different usages of prepositions, so there is not really enough space here to give a good explanation, although someone might try.