"In" has several uses: 1) To signify being in a place (this one is the most common) For example, "I am in Paris" or "I am in my bedroom." 2) To signify being enrolled in something For example, "I am in math." 3) Used with the word "love" to signify romantic love. For example, "I am in love." "On" is used to signify being above something. For example, "My computer is on my bed" or "My cat is on the dresser." It is also used to signify taking responsibility for something. For example, "I'm on it." "At" is used similarly to "in" in the first example. However, the 2 words are not interchangeable. "At" refers to a certain location, but you may not necessarily be "inside" that location. For example: "I am at the stadium" It also is used when referring to an event, such as a concert. For example: "I am at the Elton John concert." Incorrect usage: I am at America. Correct usage: I am in America. Incorrect usage: The guitar is in the couch. Correct usage: My guitar is on the couch. Incorrect usage: She is in the Katy Perry concert. Correct usage: She is at the Katy Perry concert.