We use ERA when we want to talk about someone's characteristic which she doesn't have anymore. Here an example: " A menina era tímida, mas agora ela é extrovertida" We use ESTAVA or TAVA(informal) when we want to talk about the way or state the person was, the location she was or something she was doing. Example: "Eu estava doente na semana passada" "Eu estava no shopping" "Eu estava estudando"

In Portuguese the verb Ser e Estar (to be in English) here similar mean. The verb SER represents the existence of someone, something in the time or space. In the other hand, verb ESTAR means a presente situation.

Eu era jovem - I was young Eu es(tava) lá - I was there We brazilians cut a lot of words :( Cê sabia? > Você sabia? A gente tava > Nós estávamos Vá dizê > Vai dizer Etc...