Learn Italian: Quando ha smesso di piovere, siamo usciti di casa. - When it stopped raining, we went out of the house.

Quando (when) is often used to introduce a specific, completed event that happens at a point in time, which is why it uses the passato prossimo here. If we wanted to describe an ongoing background action like "while it was raining", we would use mentre (while) with the imperfetto: mentre pioveva.

Why does the first half use avere (ha smesso) but the second half uses essere (siamo usciti)?

The auxiliary verb depends on the action. Smettere (to stop) takes avere. However, uscire (to go out) is a verb of movement, which requires essere. Because it uses essere, the ending of uscito must also change to match the plural "we" subject, becoming usciti.

Why is there a di before piovere?

In Italian, when the verb smettere (to stop) is followed by another action, you must connect them with the preposition di. So, "to stop raining" becomes smettere di piovere.

What is the infinitive of smesso?