1. There are six horses living inside the barn.

barn

 2. Something is living inside this cave, but he’s not sure what it is.

cave

 3. There’s a lot of junk inside my desk.

inside desk

 4. The inside of their house is really beautiful.

("Inside" in this example is used as a noun.)

furniture inside
 5. The medicine is inside the syringe.
medicine
 6. She doesn’t want to work inside the office today because it’s so nice outside. woman talking
 7. Most of the meetings at her company are held inside this conference room. office
 8. The fire inside this stove is really hot.
stove
 9. When the red flag goes up on the mailbox, that means there’s mail inside the mailbox. mailbox