To be in work = to have a job Example: "It's hard to find a job these days. My brother is the only person in my family who's in work. Everyone else is unemployed." To be at work = to be in the place where you work eg office, factory Example: "Is Tom home?" "No, sorry. He's at work at the moment. He'll be back around 6 o'clock."

what is the difference? what I have to use "in" or "at" and why? I will give you some examples I went swimming in the pool. (Meaning I went into the water and started to swim) My socks are in my drawer. My friends are at the mall. (They are not in/on/on top of/underneath) You are at the beach. (You are there but it doesn't describe what you are doing there just that you are there)