Questions & Answers about Ci siamo lasciati perché lui non era fedele.
Why do we use ci here? Isn't lasciare just 'to leave'?
Why is the helping verb siamo instead of abbiamo?
Why use era instead of è stato for 'was'?
era is the imperfect tense. We use the imperfect here because 'being faithful' (or unfaithful) describes an ongoing state or personality trait in the past. If we used the passato prossimo (è stato), it would sound like he was unfaithful just once for a specific, isolated moment.
Why include the pronoun lui if Italian usually leaves pronouns out?
Italian drops subject pronouns when the context is obvious, but often keeps them for contrast or clarity. Since the first half of the sentence is about 'we' (ci siamo), using lui clearly shifts the focus to emphasize that he was the one who was not faithful.