In Portuguese there are several ways to refer to something that happened in the past, each with varying shades of meaning. The imperfect indicative is used when talking about continuous or ongoing action in the past. Something that used to happen or, would (always) happen. In english we say 'I used to work at Banco do Brasil.' In Portuguese it's:
'Eu trabalhava no Banco do Brasil.'
Or, 'When I was younger I would always drink a few beers before going to a party.'
'Quando eu era mais jovem eu sempre bebia umas cervejas antes de sair pra festa'
Things that used to happen or, would (always) happen, entendeu?
> Eu sempre escutava música antes de dormir.
> Eu comia naquele restaurante antes, mas agora tá ruim.
You should learn the Portuguese imperfect conjugation only after you are comfortable with the preterit ("simple past") tense. It's the second most popular tense and you will hear it all the time, mainly when telling a story. You're probably using this tense already - when you conjugate estar into estava. That's because you're saying that something was happening when... Most students start using estava before knowing that this is the imperfect indicative.
-ar regular verbs
|I used to work||eu trabalh||ava|
|he used to work||ele trabalh||ava|
|we used to work||nós trabalh||ávamos|
|they used to work||eles trabalh||avam|
-er and -ir regular verbs
|I used to eat||eu com||ia|
|he used to eat||ele com||ia|
|we used to eat||nós com||íamos|
|they used to eat||eles com||iam|