We have already spoken about the preposition in. We have already seen that in Italian the preposition in is used in relation with countries and regions and to describe a movement towards a place and the residence at a place. All other languages describe the movement to a place with another preposition than the residence at some place, but in Italian the same preposition is used in both situations.
Vado in Francia. = I go to France.
Sono in Francia. = I am in France.
And we have already seen as well that the distinction between movement and residence doesn' t exist either in relationship to cities, but that in this case the preposition a is used.
Vado a Roma. = I go to Rom.
Sono a Roma. = I am in Rom.
Furthermore we have seen (and mentioned the exceptions as well) that the preposition in is to be used if we don't talk about a concrete building but about the function of a building. In the following sentences the noun stands once for a function and once for a concrete building.
Non vado in discoteca da tempo.
I haven' t gone to the disco for long time.
La prima sera vado alla discoteca Magrib, una delle più "in" della città e che anche il tassista Andrew mi aveva consigliato nei suoi numerosi "trust me".
The first evening I go to the disco Magrib, one of the most hip of the city and which has been recommended to me as well by the taxi driver with his numerous "trust me".
We remember that in the case of scuola, casa, cinema, parco we usea although the important thing is the function and not the building.
Vado a scuola. = I go to school.
Vado al cinema. = I go to the cinema.
Vado al parco. = I go to the parc.
One can ask oneself what is the difference between in and dentro.
Le sigarette sono nella scatola.
Le sigarette sono dentro la scatola.
The cigarettes are in the box.
In the sentences above there is no difference at all. If something is really inside something else both prepositions, in as well as dentro, can be used without a change in the meaning. And in Italian both can be used as well together with a pronoun.
Ma dentro di lui, non c'era nessuna gioia.
Ma in lui, non c' era nessuna gioia.
= But inside him there was no joy.
Used as prepositions dentro and in have an intersection and in can be substituted by dentro if an object is really inside another one.
The situation changes if dentro is used as an adverb and used as an adverb dentro can't be substituted by in. In is always a preposition and can never be used as an adverb.
Let's go in.
Lo metto dentro, cioè, in un contenitore pieno d'acqua, la forza di gravità lo attira verso il fondo.
I put it in, more accurate in a box of water, the gravity draws it to the ground.
Dentro can be used as well to describe temporal relationships.
Sono stato dentro un' ora.
= I was inside for one hour.
Sono rimasta dentro un'ora e pensavo di soffocare.
= I remained inside one hour and I believed to suffocate.
In this case dentro is a pronominal adverb corresponds to the English inside. It has characteristics of an adverb (it is a local adverbial adjunct, describes where the thing is put into) and it has characteristics of a pronoun, it stands for something. If we don't know what inside (in the book, the box, etc.) stands for the sentence doesn't make any sense.