Learn Hardcore Italian: Ieri sera avevo già chiuso la porta quando è arrivato il corriere. - I had already closed the door when the courier arrived.

Italian uses the trapassato prossimo (avere/essere in imperfetto + past participle) to describe an action completed before another past action. Here, the door-closing happened first, then the courier arrived. So you need avevo chiuso (“I had closed”) rather than ho chiuso (“I closed”/“I have closed”).

What role does già play in the sentence and where can it go?

Già means “already.” In compound tenses it normally sits between the auxiliary and the past participle: avevo già chiuso. You can also place it before the auxiliary (già avevo chiuso) for emphasis or, less commonly, after the participle (avevo chiuso già la porta), though the first option is most natural.

Why is quando è arrivato il corriere in the passato prossimo and not the imperfetto?

The imperfetto describes ongoing or habitual past actions/background. The courier’s arrival is a specific, punctual event—a completed action—so Italian uses the passato prossimo (è arrivato).

Is it mandatory to put Ieri sera at the beginning? What happens if I move it?

No, it isn’t mandatory. Ieri sera (“last night”) is a time adverbial that can go at the beginning, after the verb, or even at the end:
• Ieri sera avevo già chiuso…
• Avevo già chiuso la porta ieri sera…
• Avevo già chiuso ieri sera la porta…
The basic meaning stays the same, but the emphasis shifts slightly depending on position.

Why are there no subject pronouns before avevo or è?

Italian is a pro-drop language: verb endings clearly indicate person and number, so subject pronouns (io, tu, lui/lei) are often omitted. You’d add io or lui only for contrast or emphasis (e.g. Io avevo già chiuso).

Can I invert the clauses and say Quando è arrivato il corriere, avevo già chiuso la porta?

Yes. In Italian you can freely swap main and subordinate clauses. The meaning remains: the door was already closed when the courier arrived. The only real change is a slight shift in focus (you emphasize the courier’s arrival by putting it first).

Could I say La porta l’avevo già chiusa instead of avevo già chiuso la porta?

Absolutely. Using a “clitic doubling” structure (object pronoun + noun) you get more emphasis on “la porta”:
La porta l’avevo già chiusa quando è arrivato il corriere.
Both forms are correct; the doubled-object form highlights the door.

What’s the difference between chiudere la porta and serrare la porta?

Chiudere la porta simply means “to close/shut the door.” Serrare la porta implies securing it—locking, bolting, or fastening it tightly. If you want “I had locked the door,” you could say avevo già serrato la porta (or avevo già chiuso a chiave la porta).