I know all the rules about how to form the articulated prepositions in Italian, so I don't have to understand how to construct them, but rather I have a doubt on when to use the simple and when to use the articulated one:
Vado a scuola.
Why don't we say "alla scuola", but we use only the simple preposition? In what other cases does something like this happen?
As stated by Maria Cristina Peccianti in Grammatica italiana per stranieri (Giunti Editori, 2013) and by Federica Colombo in Grammatica e pratica della lingua italiana per studenti stranieri (edizioni ELI, 2006):
If the place is the name of a country, state, continent, region or big island, we always use the simple preposition "in": Mia figlia è andata in Francia. Mi piacerebbe andare in Liguria. Voglio andare in campeggio in Corsica. Sei stato in Africa?
However, if such names are in plural or if they are followed by a specification (e.g. an adjective), you must use the articulated prepositions, that is, "nel","nello", "nell'", "nella", "nei", "negli", "nelle". For instance: Non sono mai stato nella Francia meridionale.Voglio andare negli Stati Uniti.
If the place is the name of a small island, we use the simple preposition "a": Andiamo a Capri.
If the place is the name of a city or town, we always use the simple preposition "a": Il 2 luglio andremo a Siena.
If the place is the name of a street or square, usually we use the simple preposition "in": Laura abita in via Buozzi.
If the place is the name of a store or shop, usually we use the simple preposition "in": Devo andare in farmacia.
However, if the name of such store or shop is followed by some specification, one must use the articulated preposition "a" or the articulated preposition "in": Ho comprato queste pillole alla farmacia dei Quattro Cantoni.
When the place refers to people, we use always the preposition "da", simple or articulated, following the rules of use of articles: Starò da Antonio due giorni.
There are lots of usual expressions of place with which one uses the prepositions "a" or "in", simple or articulated, which don't follow any rule or regularity. Some examples stated in the book by Peccianti are: