When the grammatical subject is irrelevant or not known:
Le forme impersonali

Look at these sentences:

"Do you drive on the right or on the left in Italy?" "You drive on the right."
"Where do you find hippopotamuses?" "It depends on where you leave them."

They contain "impersonal you" - it can be used both in the questions and in the answers because the questions do not enquire about the person spoken to, but about general facts. Here are the It. translations:

"Si guida sulla destra o sulla sinistra in Italia?" "Si guida sulla destra."
"Dove si trovano gli ippopotami?" "Dipende da dove si lasciano."

Impersonal forms with the pronoun "si" are frequently used where Eng. uses the passive voice without an agent:

"Qui si parla inglese" ( English [is] spoken [here]).
"In Italia si fabbricano ottime scarpe" ( Excellent shoes are made in Italy).
"Le famose rocce marine di Capri si chiamano faraglioni" ( The famous sea-rocks in Capri are called faraglioni).
"Non si fa così!" ( That's not done!)

Other impersonal subjects in Eng. are "one, we, they, someone"... Examples:

"Non si sa mai" ( One never can tell).
"Sbagliando si impara" ( We learn by making mistakes).
"Si produce molto riso in Cina" ( They produce a lot of rice in China).
"Si dice che D. T. stia per divorziare" ( Someone says D. T. is about to divorce).