Learn Italian: Mentre guardavamo l'arcobaleno, è uscito il sole. - While we were looking at the rainbow, the sun came out.

Mentre means "while," which naturally sets up a continuous, ongoing background action. In the past tense, we use the imperfetto to describe these "was doing" or "were doing" actions.

Why is the second verb (è uscito) in the passato prossimo?

It represents a sudden, completed action that happened during the ongoing background event. This classic combination—imperfetto for the background scene, and passato prossimo for the interrupting event—is how you tell stories in Italian.

Why does uscito use è as its helper verb instead of ha?

The verb uscire (to go out / to come out) is a verb of movement. In the passato prossimo, verbs that describe moving from one place to another, or changing state, almost always take essere instead of avere.

Why does the sentence say è uscito il sole instead of il sole è uscito?

While il sole è uscito is not grammatically wrong, placing the subject after the verb is very natural in Italian for sudden appearances, weather events, or when introducing new information to the scene. It puts the focus on the surprise: the sun appeared!