Drop the infinitive ending and add -ava, -avas, -ava, -ávamos, -avam to -ar verbs, or -ia, -ias, -ia, -íamos, -iam to -er and -ir verbs; only ser (era), ter (tinha), vir (vinha) and pôr (punha) are irregular. Use it for what used to happen repeatedly, for describing the past (weather, age, mood, scenery), and for an action already in progress — while the pretérito perfeito reports the single finished event that cuts in.
Examples
- Quando era criança, jogava futebol todos os dias.
- When I was a child, I used to play football every day.
- Chovia e não havia ninguém na rua.
- It was raining and there was nobody in the street.
- Eu lia o jornal quando o telefone tocou.
- I was reading the newspaper when the phone rang.
- Nós íamos à praia sempre que fazia calor.
- We used to go to the beach whenever it was hot.
The mistake to avoid
Learners reach for the pretérito perfeito for background and repeated past ("ontem lia o jornal quando ele chegou" / "quando era criança joguei futebol"), but habits and scene-setting take the imperfeito and only the interrupting event takes the perfeito.
Where this fits in
The imperfect: past habits & descriptions, Portuguese B1: Use the pretérito imperfeito to talk about how things used to be, describe the past, and narrate ongoing actions interrupted by another event.