अंदर कोई है

You use this phrase to describe the status of a space. You don't need complex grammar here. Just state it as a fact. If you are inside a room and someone knocks, you can shout this out. If you are standing outside a bathroom and your friend tries to enter, you say it to stop them. It works for rooms, cars, or even small shops.

Use it whenever you need to indicate occupancy. It is perfect for public restrooms. Use it at the office when a meeting room is taken. It is also useful at home if you have guests. If you see a light on in a dark room, you might ask it as a question: अंदर कोई है? (Is someone inside?).

Do not use this for abstract concepts. It is strictly for physical spaces. Don't use it to mean "someone is in my heart" or "someone is in my thoughts." That would sound very strange! Also, avoid using it if you know exactly who is inside. If your brother is in the room, say भाई अंदर है (Brother is inside) instead. Using कोई (someone) implies you might not know who it is or it doesn't matter.

In India, privacy boundaries can be fluid. People might not always knock before entering a room at home. Saying अंदर कोई है is a polite but firm way to establish a boundary. In busy public spaces, it’s a necessary survival phrase. It’s the verbal version of a "Do Not Disturb" sign.

You can turn it into a question by raising your pitch at the end. You can add शायद (perhaps) at the beginning if you aren't sure: शायद अंदर कोई है. If you want to be more formal, you might say अंदर कोई मौजूद है (Someone is present inside), but that is quite rare in daily talk. Stick to the simple version for the best results.