What the si passivante does
Italian signs rarely carry a passive with essere. Instead of I biglietti sono venduti qui you usually read Qui si vendono i biglietti. This construction is called the si passivante — passive si.
The thing that something is done to is the subject of the sentence, and the verb agrees with it. English can say They sell flats, where the verb stays fixed, or Flats are sold, where it agrees: a flat is sold, flats are sold. Only the second matches the Italian analysis.
The difference made audible
Si vende un appartamento.
One flat is for sale.
Si vendono appartamenti.
Flats are for sale.
The verb agrees with the thing
Always ask: what is the thing being acted on? If it is singular, take the third person singular. If it is plural, take the third person plural. Word order changes nothing: the thing usually comes after the verb, but it is still the subject.
In real use
Qui si parla italiano.
Italian is spoken here.
In questo hotel si parlano quattro lingue.
Four languages are spoken in this hotel.
Da noi si servono solo prodotti locali.
Only local produce is served at our place.
Non si accettano carte di credito.
Credit cards are not accepted.
With no thing in the sentence the verb stays singular: In Italia si vive bene, D'estate si lavora meno. You can tell whether a thing is there by going plural: in Qui si parla italiano the agreement is invisible, in si parlano quattro lingue it is not.
In the passato prossimo
In compound tenses the si passivante always takes essere as its auxiliary — including with verbs that normally want avere — and the participle agrees with the thing in number and gender: not Si ha venduto la casa, but Si è venduta la casa. English never asks you for either, so learn both as mechanics.
Present and passato prossimo side by side
| Presente | Passato prossimo | English (past) |
|---|---|---|
| Si vende il libro. | Si è venduto il libro. | The book was sold. |
| Si vendono i libri. | Si sono venduti i libri. | The books were sold. |
| Si mangia la torta. | Si è mangiata la torta. | The cake was eaten. |
| Si mangiano le olive. | Si sono mangiate le olive. | The olives were eaten. |
Si sono venduti tutti i biglietti in due ore.
All the tickets were sold in two hours.
Si è presa una decisione difficile.
A difficult decision was taken.
Where you will run into it
Small ads, signs, recipes and house rules are full of it. English shortens such notices (Rooms to let, No pets allowed); Italian keeps a full sentence with a finite verb, and the verb still counts the things.
Si affittano appartamenti.
Flats to let.
Si cercano collaboratori.
Staff wanted.
Si taglia la cipolla a fettine.
Slice the onion thinly.
Si aggiungono due uova.
Add two eggs.
Non si ammettono animali.
No pets allowed.
On signs you also see Affittasi and Vendesi: the si is attached to the end of the verb, a short form of si affitta and si vende.
