Learn Hardcore Italian: La porta si richiude lentamente per il vento. - The door closes again slowly because of the wind.

In this sentence, si is the reflexive pronoun in the pronominal verb richiudersi, marking that the subject (the door) closes itself. Without si, richiudere would be transitive and require an object.

Why use richiudere instead of chiudere here?

The prefix ri- means “again.” richiudere literally translates as “to close again.” It indicates that the door, after being open or ajar, is being shut once more by the wind.

Is this construction passive?

No. Although Italian can form a passive with si, here si belongs to the pronominal verb richiudersi, not to the passive. A true passive would be La porta è chiusa dal vento, using essere + past participle and da for the agent.

Why is per used in per il vento? Could we say dal vento?

To indicate cause, Italians usually use per or a causa di. So per il vento means “because of the wind.” dal vento would be reserved for passive sentences to introduce an agent, which is not the case here.

What does lentamente do in the sentence, and can its position change?

Lentamente is an adverb modifying si richiude (“closes slowly”). It normally follows the verb, but you can also place it before for emphasis: La porta lentamente si richiude per il vento.

Can we replace lentamente with another adverb like piano?

Yes. Piano is a common colloquial adverb meaning “slowly”: La porta si richiude piano per il vento. It’s slightly less formal than lentamente.

How do you conjugate richiudersi in the present tense?

The present indicative of richiudersi is:
io mi richiudo
tu ti richiudi
lui/lei si richiude
noi ci richiudiamo
voi vi richiudete
loro si richiudono