Quick answer
se habla español · se venden pisos
A way to say something happens without saying who does it — Spanish’s everyday replacement for the passive.
The verb follows the noun
If what follows is singular, the verb is singular. If it is plural, so is the verb.
The general-you use
With no noun after it, se plus a third-person singular verb means "one", "people" or the vague English "you".
Instead of the passive
Spanish has a passive with ser, but it sounds formal and appears mostly in journalism. In speech, se does the work.
Memory trick
Look at the noun right after the verb. It is the subject, and it decides whether the verb is singular or plural.
Typical mistakes
Se vende pisos.
Se venden pisos.
Pisos is plural, so the verb must be too. This slip is common even on real shop signs.
Aquí es hablado español.
Aquí se habla español.
The ser passive is grammatical but stilted here. Everyday Spanish uses se.
Se puede fumar? No, no puede.
¿Se puede fumar? No, no se puede.
The se belongs to the construction and cannot be dropped in the answer.
Check yourself
En la tienda ___ libros usados.