Dentro can be used to describe the residence at a place and to describe the movement toward a place.
description of the residency
I am in / inside.
description of a movement
I go in / inside.
The same distinction as in English between in and inside exists in Italian between in and dentro. Dentro is used if an object is completely surrounded by something. But as an adverb you can only use dentro (wrong: Vado in. correct: Vado dentro.).
substitution possible: Completely surrounded
Dentro la caverna si sono trovate tracce di una cultura preistorica.
Inside the cave were found sign of a prehistoric culture.
Nella caverna si sono trovate tracce di una cultura preistorica.
In the cave were found signs of a prehistoric culture.
substitution impossible: Not completely surrounded
I bambini giocano nel giardino.
not: I bambini giocano dentro del giardino.
The children are playing in the garden.
not: The children are playing inside the garden.
Neither dentro nor inside can be used in metaphorical sense.
In questa situazione era il meglio che si poteva fare.
not: Dentro questa situazione era il meglio che si poteva fare.
In this situation it was the best thing to do.
not: Inside this situation it was the best thing to do.
We have talked already about the preposition dentro in the chapter prepositions.