Learn Italian: Che cosa c'è dentro?

Because in this sentence, dentro is acting as an adverb (describing a location) rather than a preposition linking to a noun. If you were saying "inside the box," you would add the noun: dentro la scatola. But when you just say "inside" without mentioning the specific container, dentro stands entirely alone.

The translation is "What is inside?". Why do we use c'è ("is there") instead of just è ("is")?

In English, we can simply ask what "is" in a location. In Italian, when you ask about the presence or existence of something in a specific place, you must use the verb esserci (ci + essere). Italian is literally asking "What is there inside?". Saying Che cosa è dentro? would sound incomplete or unnatural.

Can I just say Cosa c'è dentro? instead of Che cosa c'è dentro?

Yes, absolutely! To say "what" in a question, you can use che, cosa, or che cosa. They all mean the same thing and are usually interchangeable. Cosa is very common in everyday spoken Italian, while che cosa is slightly more traditional and formal.

Could I use the preposition in instead of dentro here?