La porta si richiude da sola.

The prefix ri- usually adds the idea of again or back.

• chiudere = to close
• richiudere = to close again, to close back up

So si richiude suggests the door was open and then closes again.

Here si is part of the verb and shows that the action happens to the subject itself, rather than being done by someone else.

• Chiude la porta = he/she closes the door
• La porta si chiude = the door closes
• La porta si richiude = the door closes again / closes back up

In English, we often just say the door closes, but Italian commonly uses this reflexive-looking structure with things like doors, windows, shops, etc.

Not exactly in the fully literal sense. Grammatically it looks reflexive, but in practice it often works more like an intransitive/pronominal verb: the subject undergoes the action.

So La porta si richiude is a normal way to say that the door closes again. It does not necessarily imply that the door has intention or is actively doing something on purpose.

The h keeps the c hard before i.

Compare:

• ci = pronounced like chee
• chi = pronounced like kee

Since chiudere has a hard k sound, the spelling keeps that sound:

• chiudere
• richiudere
• richiude

Without the h, the pronunciation would change.

Because richiude is a conjugated verb in the third person singular present.

The subject is la porta, which is singular, so the verb must match it:

• io richiudo
• tu richiudi
• lui/lei richiude
• la porta richiude → with si: la porta si richiude

richiudere is the infinitive, meaning to close again.

Da sola means by itself / on its own.

It emphasizes that no person is closing the door. The door closes again automatically or without help.

This expression is very common in Italian:

• Si apre da sola = it opens by itself
• Si spegne da solo = it turns off by itself
• La porta si richiude da sola = the door closes again by itself

Because it agrees with la porta, which is feminine singular.

Agreement here follows the noun being described:

• la porta → feminine singular → da sola
• il cancello → masculine singular → da solo
• le porte → feminine plural → da sole
• i cancelli → masculine plural → da soli

Yes, da sé is possible and also means by itself. However, da sola is often more natural and common in everyday speech for this kind of sentence.

Very roughly:

• da sola = by itself, on its own
• da sé = by itself, of its own accord

Both are correct, but da sola usually sounds more conversational here.

The subject is La porta.

That is why the verb is singular:

• La porta si richiude
• not Le porte si richiudono

Even though there is a si, the subject is still la porta.

It can do either, depending on context.

Italian present tense often covers both:

• right now: the door is closing again by itself
• general/habitual: the door closes again by itself

Without extra context, the sentence can be understood either way.

Yes. That is a very natural alternative.

• si richiude = closes again / closes back up
• si chiude di nuovo = closes again

The version with ri- is more compact. The version with di nuovo is a bit more explicit. Both work.

Putting da sola at the end is the most neutral and natural order here. It adds the final piece of information: the door closes again — by itself.

Italian word order is flexible, but this version sounds very standard:

• La porta si richiude da sola.

Other orders are possible for emphasis, for example:

• Da sola, la porta si richiude.

But the original sentence is the most ordinary one.

Yes. Da sola often suggests exactly that kind of situation: the door closes on its own because of a mechanism, hinge, spring, airflow, or similar cause.

So the sentence is very natural for describing how something functions, not just a one-time event.