Imperfetto or Passato Prossimo?

Italian tells a story in the past with two tenses, and the context decides which one you need.

Use the imperfetto for the background: what was going on, habits, descriptions, age, weather and feelings. Typical cues: mentre, da bambino, ogni estate, la domenica.

Use the passato prossimo for completed events that move the story forward. Typical cues: ieri sera, l'anno scorso, all'improvviso, tre volte, per due ore.

• An action in progress is interrupted: Mentre leggevo, è suonato il telefono. (While I was reading, the phone rang.)
• For an action in progress you can also use stare + gerundio: stavo leggendo.
• A finished period with a stated length takes the passato prossimo: Ha lavorato lì per trent'anni.