Talking without a subject
Very often it does not matter who does something — the doer is unknown, general, or simply beside the point. For that Spanish puts the pronoun se in front of the verb and names no subject at all. In everyday Spanish this is far more common than the passive with ser.
English has no dedicated word for this and uses a whole shelf of workarounds: they say it will snow, you can't smoke in here, people eat late in Spain, Spanish is spoken here. Spanish uses a single construction for all of them, which is easier once you stop hunting for a word-for-word match.
The effect in two sentences
Aquí se habla español.
Spanish is spoken here. / They speak Spanish here.
Se vive bien aquí.
Life is good here. / People live well here.
Se impersonal, the general you
With the se impersonal there is no thing for the verb to agree with, so it stays in the 3rd person singular, always. It is typical with verbs that take no object (vivir, cenar, trabajar), before a que clause (se dice que) and with se + modal + infinitive (se puede, se debe).
Se impersonal in sentences
En España se cena muy tarde.
In Spain people have dinner very late.
Se dice que va a nevar mañana.
They say it's going to snow tomorrow.
¿Se puede fumar aquí?
Is smoking allowed here?
Se pasivo, the everyday passive
When a verb that takes an object (vender, alquilar, necesitar) has a thing with it, that thing becomes the grammatical subject and the verb agrees with it in number: se vende una casa, se venden pisos. The technical name is pasiva refleja.
English intuition misleads you here. Se venden pisos feels like “they sell flats”, with pisos as the object, so you want a singular verb. Read it as the passive “flats are sold” and the plural verb stops looking strange.
Se pasivo in sentences
Se alquila una habitación.
A room is for rent.
Se alquilan habitaciones.
Rooms to let.
En la oficina se hablan tres idiomas.
Three languages are spoken in the office.
Where you will meet SE sentences
On signs and in rules, SE constructions are everywhere. English signs mostly manage without a verb at all (For sale, No entry), while Spanish keeps one. These are the phrases worth recognising on sight:
Signs and small ads
Se alquila
For rent
Se vende
For sale
Se busca camarero
Waiter wanted
Rules and prohibitions
Se prohíbe fumar.
Smoking is prohibited.
Se ruega silencio.
Silence is requested.
No se permite la entrada.
No entry.
