Learn Spanish (Latin America): Las llaves están dentro de la casa.

While en can mean 'in,' 'on,' or 'at,' dentro de specifically means 'inside' or 'inside of.' Using dentro de emphasizes that the keys are physically enclosed within the house, rather than just being somewhere at the general location.

Does dentro always need to be followed by de?

You need de when you are stating what the object is inside of. Because we are specifically saying the keys are inside the house (la casa), we must use dentro de. If the house was already mentioned and understood from context, you could just say Las llaves están dentro (The keys are inside) without the de.

Could I use adentro de instead of dentro de here?

In many parts of Latin America, you will often hear people say adentro de la casa to mean the exact same thing. Traditionally, adentro implies movement (going inside) and dentro is for location (being inside), but in everyday spoken Latin American Spanish, they are frequently used interchangeably.

Quick refresher: why are we using están instead of son for 'are'?