Thanks for response. I am surprised. I thought that "estava" was "Indicativo Imperfeito". And I thought that "Indicativo Imperfeito" was not used for one single and finished action in the past. ?? I thought that "fui" could both be "Pretérito Perfeito do Indicativo" of "ir" and "ser"?
Yes, but "fui" is not used to mean you were at some place, but "eu já fui magro", "eu fui seu amigo". To places you use "estar" (that is not permanent), so you use estive/estava. The context will show you the best one to use. Here we have no context so both are suitable.