You would absolutely say "I'm at the beach" or "I'm in the mall" but you could also use the other two (although in these two sentences "in" would sound a little odd to me). Prepositions can be really hard so here is a general outline of what each are used for: "On" Used to express a surface of something: I put an egg on the table. Used to specify days and dates: I was born on January 15th Used to indicate a device or machine, such as a phone or computer: He is on the phone right now. Used to indicate a part of the body: The stick hit me on the shoulder. Used to indicate the state of something: The building is on fire. "At" Used to point out specific time: I will meet see you at 12 p.m. Used to indicate a place: There is a party at the club house. Used to indicate an email address: Please email me at [email protected]. Used to indicate an activity: I am good at drawing a portrait. "In" Used for unspecific times during a day, month, season, year: The new semester will start in March. Used to indicate a location or place: I am currently staying in a hotel. Used to indicate a shape, color, or size: This painting is mostly in blue. Used to express while doing something: In preparing for the final report, we revised the tone three times. Used to indicate a belief, opinion, interest, or feeling: We are not interested in gambling. I got my examples from http://www.talkenglish.com/ so maybe go over there at take a look!

Yes. "I'm at the mall" and "I'm at the beach." are fine. I am IN the process of answering you. My mind is working ON your question. I am AT the Italki website. I may be IN trouble if I make a grammar error here. Your question appears ON my computer monitor. If I cannot think of answers for you, I will be AT a complete loss. AT my house, I am always willing to offer coffee or tea to visitors. You are welcome to sit ON my couch. I live in Denver, Colorado, USA. Let's see what is ON television. Let's see what is playing ON the radio. There are two police cars AT a house up the street Would you like sugar IN your coffee? There is someone ON the phone asking for you. Send me the information IN an e-mail. I will meet you AT the coffee shop at the intersection of Logan and Alameda streets. AT the same time that I am answering your question, someone else will probably be answering your question also. What do you see ON the horizon. I will meet you AT your house. .

Thanks for your answers. They were very helpful ;)