The passato prossimo tells you what happened; the imperfetto describes how things were around it. If the event has a beginning and an end inside the story you're telling, use the passato prossimo; if it's the backdrop, was ongoing, or used to repeat, use the imperfetto.
The test, in one sentence
Ask whether you're moving the story forward or furnishing it. "Yesterday I ho incontrato Marco" moves it: there's a before and an after. "It era late and it pioveva" furnishes it: it describes the scene the event happens in.
They sit happily in the same sentence, and that's where the test shows best: *Camminavo per strada quando ho visto Marco* — walking is the backdrop, seeing is the event that interrupts it.
Three cases where the imperfetto is compulsory
Descriptions: what a person, place, the weather, the time or an age was like. *Aveva vent'anni e abitava a Roma.*
Habits: what used to happen, repeatedly. Da bambino giocavo a calcio ogni domenica. Watch for sempre, ogni volta, di solito.
Ongoing actions, interrupted or not: Mentre studiavo, è arrivato un messaggio.
The case that trips everyone up
Some verbs change meaning with the tense — not an exception, just the same test applied consistently. *Sapevo la risposta means you already knew it; Ho saputo la risposta* means you found it out at that moment.
The same goes for conoscere — *conoscevo Marco (I'd known him a while) versus ho conosciuto Marco (I met him) — and for dovere, potere, volere: dovevo andare leaves open whether you went, ho dovuto andare* says you did.
Related questions
How do I know when to use the imperfetto?
If you can swap the verb for "was doing" or "used to do" without changing the meaning, it's the imperfetto. If the event is a single completed point, it's the passato prossimo.
Is the imperfetto the same as the English past continuous?
It overlaps but is wider. The imperfetto also covers habits ("used to") and descriptions, where English would use a simple past.
Updated 2026-07-29