Ser is irregular in the Imperfect Indicative tense.
✻ The Imperfect tense is used when talking about continuous or ongoing action in the past. Something that used to happen or, would (always) happen.
eu ➜ Ela era uma criança tranquila. (She was a peaceful child.)
você, ele, ela ➜ Era uma vez. (Once upon a time.)
nós ➜ Nós éramos vizinhos. (We used to be neighbors.)
vocês, eles, elas ➜ Vocês eram estudiosos na escola. (You were studious in school.)