Learn Italian: La pioggia aveva corroso il tetto, perciò l'acqua cadeva dentro. - The rain had corroded the roof, so the water was falling inside.

Our current sentence uses perciò to show cause-and-effect, but if you wanted to express a contrast, you could use the lesson's grammar. You would pair a concessive conjunction directly with the past participle corroso. For example: Sebbene corroso dalla pioggia, il tetto non è crollato (Although corroded by the rain, the roof didn't collapse).

What is the infinitive of corroso, and is this a common past participle pattern?

The infinitive is corrodere (to corrode). Verbs ending in -dere often have irregular past participles ending in -so. You have likely seen this pattern before with verbs like ridere (riso), decidere (deciso), and esplodere (esploso).

Why are we pairing the trapassato prossimo (aveva corroso) with the imperfetto (cadeva)?

This combination perfectly establishes the timeline of past events. Aveva corroso (had corroded) is the past perfect, showing that the corrosion happened first. Cadeva (was falling) is in the imperfect because the water falling inside was an ongoing, continuous state that resulted from that earlier damage.

Could I use a causal conjunction like poiché or siccome instead of perciò?