Impersonal and Passive sentence, what is the difference?
This reference article has been created by Lazarus, grammar expert on this forum
What is an impersonal sentence?
Impersonal sentences are those where the agent does not exist, or it has been hidden or ignored. This effect can be achieved in many ways.
From the interpretation of the sentence. One way is to use a passive with "se" are which have a subject, but no agent is mentioned. The subject must agree with it: "Se vende un piso", "Se venden dos pisos" (See below for more details). The normal passive achieves the same impersonal effect: "El museo ha sido restaurado". Another method is to use "tú" or "uno": "En esta universidad tienes que estudiar mucho / ...uno tiene que...", where that "tú" is not the person you are talking to, but anyone.
From the interpretation and the syntax. Sentences using an impersonal "se" don't have a subject (at all), and no agent can be found either. The verb is always in the third person singular (See below for details). Sentences in the 3rd person plural can be used to achieve a similar effect: "Me han robado el bolso", where the subject is not "ellos", but God-knows-who. In this case, the agent is completely unknown, and "ellos" cannot be mentioned, or the meaning of the sentence changes, and you'd be referring to a particular group of people that you know of. Other sentences with non-finite forms, like "Es preferible votar" have a subject (votar), but "votar" doesn't have one, so there is no specific agent to be found either.
Syntactically, verbs like "llover", "nevar", "son (las tres de la tarde)", "hacer (calor/frío/...)", "hay (algo)", "hace (un año que no te veo)", "hay (que trabajar más)", "parece (que...)" have no subject or agent. There are a few others, like "Falta (de todo)", "Da (miedo verlos pelear)"...
Differences between impersonal "se" and passive with "se"
While in plural it is obvious that it cannot be an impersonal "se" (these must be always in 3rd person singular), in singular it is not so clear, and sometimes it is virtually impossible to differentiate them. However, in terms of classification, according to the RAE, the impersonal "se" is only used with
(1) direct objects that refer to specific people, and therefore they require the personal "a": "Se entrevistó a los jugadores"
(2) intransitive verbs: "Se vive bien, se come bien..."
The passive reflexive verbs are used only with transitive verbs, except when they refer to specific people and the direct object has no personal "a".
