Learn Italian: Speravo che la perturbazione fosse già passata. - I hoped that the weather front had already passed.

The main verb (Speravo) is in a past tense. Because the action of the weather front passing happened before the action of hoping, Italian requires the Pluperfect Subjunctive (an imperfect subjunctive auxiliary plus a past participle) to show that it was already a completed event.

What makes this sentence require the subjunctive mood in the first place?

The verb sperare (to hope) expresses a wish, emotion, or uncertainty. Whenever you have a verb of hoping followed by che (that) and a new subject, you must use the subjunctive mood for the second verb.

Why does the past participle end in -a (passata)?

In compound tenses that use essere as the helping verb, the past participle must match the subject in gender and number. Since la perturbazione (the weather front) is feminine and singular, the participle becomes passata.

Why is già placed exactly where it is?

Short adverbs of time like già (already), ancora (yet/still), and mai (never) are typically placed right between the helping verb (fosse) and the past participle (passata), just like in English.