What is the passive se in Spanish?

The passive se (se pasiva) is a way of expressing a passive idea without ser. You put se in front of a third-person verb, and the thing acted on becomes the grammatical subject, controlling the agreement: se vende casa (a house is sold / for sale), se venden casas (houses are sold), se habla español (Spanish is spoken). No agent is named - you never say who is doing it. It is the construction Spanish reaches for in adverts, signs and notices where English would use a passive: se alquila, se necesita, se prohíbe.

What is the difference between the passive se and the impersonal se?

The passive se has a real subject - a thing - that the verb agrees with, so when that thing is plural the verb goes plural: se venden casas, se abren las puertas. The impersonal se (se impersonal) has no such subject; it means a generic one, you or people, and the verb stays resolutely third-person singular whatever follows: se dice que, se come bien aquí, aquí no se puede fumar. The quick test is agreement. If a plural noun pulls the verb into the plural, it is the passive se. If the verb stays singular no matter what, it is the impersonal se.

Why does Spanish prefer se to the passive with ser?

Because it is lighter and needs no agent. The ser-passive (la casa fue vendida) is formal and tends to appear only when you want to name who did it with por. When there is nobody to name, Spanish much prefers the passive se (se vendió la casa) or the impersonal se. They are everywhere in everyday Spanish - shop signs, notices, instructions, general statements - precisely where English would reach for a passive or for a vague they or you. See the passive with ser page for the construction these two replace.

The Passive se and the Impersonal se in Spanish

The last page on the passive with ser ended with a warning: Spanish uses that construction far less than English, and most of the time it reaches for se instead. This is that se. It is one of the most useful little words in the language, and it hides two related constructions that learners blur all the time. Sort them out and you will sound far more natural than anyone leaning on ser fue.

Two constructions, one se

Both put se in front of a third-person verb and refuse to name who did the action. The difference is what the verb agrees with.

Passive se (se pasiva): the thing acted on becomes the subject and controls the agreement. Se vende casa, se venden casas, se habla español.
Impersonal se (se impersonal): there is no such subject. The verb means a generic one, you, people and stays third-person singular whatever happens. Se dice que, se come bien aquí, aquí no se puede fumar.

Keep that split in mind and the rest is detail.

The passive se: things become the subject

In the passive se, the thing that gets acted on is promoted to subject, and the verb agrees with it just as a verb agrees with any subject. Singular thing, singular verb; plural thing, plural verb.

Se vende casa. (A house is for sale.) - one house, vende.
Se venden casas. (Houses are sold / for sale.) - several houses, venden.
Se habla español. (Spanish is spoken.) - español singular, habla.
Se hablan inglés y francés. (English and French are spoken.) - two languages, hablan.
Se abren las puertas a las nueve. (The doors are opened at nine.) - las puertas plural, abren.

You never say who does the action - the houses simply get sold, the doors simply get opened. This is the construction on every Spanish shop window and small ad: se alquila (to let), se necesita (wanted), se prohíbe (forbidden), se ruega (please). Where English uses a passive or an empty "we", Spanish uses se plus an agreeing verb.

The impersonal se: one, you, people

The impersonal se has no thing-as-subject at all. It expresses a generic doer - the English one, the vague you, the empty they or people - and because there is no subject to agree with, the verb stays third-person singular, always.

Se dice que es difícil. (They say it is hard. / It is said to be hard.)
Se come bien aquí. (The food is good here. / One eats well here.)
Aquí no se puede fumar. (You cannot smoke here.)
Se vive bien en España. (Life is good in Spain.)
Se trabaja mucho en esta empresa. (People work hard in this company.)

None of those has a plural thing controlling the verb, so it never moves off the singular. Se dice, never se dicen, when it means "they say". Se come bien, never se comen bien. The singular is the signature of the impersonal se.

Telling them apart: follow the agreement

The two overlap enough to confuse, and the test that separates them is agreement.

If there is a plural thing that pulls the verb into the plural, it is the passive se: se venden casas, se abren las puertas, se hablan dos lenguas.
If the verb stays singular no matter what follows, it is the impersonal se: se dice que, se come bien, se puede fumar, se vive bien.

Compare the near-identical pair:

Se venden libros. (Books are sold.) - libros is the subject, plural, so venden. Passive se.
Se vende bien aquí. (One sells well here / selling goes well here.) - no thing as subject, so singular vende. Impersonal se.

When in doubt, look for a noun that could be the subject. If a plural one is forcing the verb plural, you are in the passive se. If nothing is, you are in the impersonal se.

Why both beat the ser-passive

These two se constructions are why the passive with ser is so rare in everyday Spanish. The ser-passive is formal and really wants an agent named with por. When there is nobody to name - and most of the time there is not - Spanish much prefers se:

La casa fue vendida. (formal, ser-passive) becomes the everyday Se vendió la casa. (passive se).
El español es hablado aquí. (heavy) becomes Se habla español. (natural).

So treat the se constructions as the default and the ser-passive as the marked, formal exception. On a shop sign, in a notice, in a general statement, Spanish reaches for se almost every time.

Worked examples

Se venden casas en este barrio. (Houses are sold in this area.) - passive se, plural, venden.
Se habla español y se entiende inglés. (Spanish is spoken and English is understood.) - passive se, both singular patients.
Se dice que va a llover. (They say it is going to rain.) - impersonal se, singular dice.
Aquí se come muy bien. (The food here is very good.) - impersonal se, singular come.
No se puede entrar sin entrada. (You cannot get in without a ticket.) - impersonal se, singular puede.
Se abren las puertas a las ocho. (The doors open at eight.) - passive se, plural abren.

Common mistakes English speakers make

Forgetting to make the passive se agree. Writing se vende casas for several houses keeps the verb singular when the plural subject needs venden: se venden casas. If a plural thing is the subject, the verb goes plural.

Making the impersonal se agree when it should not. Producing se dicen que or se comen bien drags the impersonal se into the plural. It never moves off the singular: se dice que, se come bien. There is no plural subject, so there is no plural verb.

Confusing the two and guessing the verb. The fix is the agreement test. Look for a thing that could be the subject: if a plural one is there (se venden casas), agree with it; if nothing is (se vive bien), stay singular. Do not guess - check what, if anything, the verb has to agree with.

Reaching for the ser-passive out of habit. Translating "houses are sold here" as las casas son vendidas aquí is grammatical but stilted. Spanish says se venden casas aquí. Default to se; keep ser for formal prose and for when you are naming the agent with por.

Get the agreement split straight - plural thing pulls the passive se plural, nothing keeps the impersonal se singular - and these two cover most of the ground English hands to the passive, far more naturally than ser ever would.