SER is irregular in the Imperfect Indicative tense.
Use the Imperfect tense when talking about actions that were ongoing or habitual actions in the past, background information, or description in the past, and indirect speech.
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.)
Mixing the perfect & imperfect.
In Portuguese, the two main past tenses are used for storytelling: Pretérito Perfeito (over & done), and Pretérito Imperfeito. The two are often mixed together in a single sentence!
perfect + imperfect ➜ Pensei que era uma boa ideia. I thought it was a good idea.