The passive voice (voz pasiva)

The 'true' passive is formed with ser + past participle. The participle agrees in gender and number with the subject.

The performer of the action (the agent) can optionally be added with por.

This form is mainly used in written, formal or journalistic language (news reports, official texts); in spoken Spanish the active voice or the pasiva refleja is often preferred.

Estar + participle does not describe an action, but the state or result of it — there is no ongoing action, only a resulting condition.

Therefore 'por + agent' never appears with estar + participle: that would point back to an action.

Compare the difference between a process (ser) and a state (estar).

In spoken and general Spanish, the pasiva refleja is much preferred: se + verb in the 3rd person (singular or plural, depending on the subject).

This form is especially common when:
- the agent is unknown, unimportant or general;
- the subject is a thing (not a person).

The pasiva refleja does not take 'por + agent'.

A practical rule of thumb:

• Agent stated and important (especially in formal/written text) → ser + participle + por.
• State/result, no action → estar + participle.
• Agent unknown/unimportant, everyday language, subject = a thing → pasiva refleja (se + verb).

The passive with ser often sounds somewhat stiff in spontaneous spoken language; native speakers tend to prefer the pasiva refleja or simply the active voice.