Perfeito vs imperfeito

The short answer

Telling a story in the past means choosing, verb by verb, between two tenses. The perfeito (preterite) carries the events that move the story forward: cheguei, vi, liguei. The imperfeito (imperfect) paints everything around them: the time, the weather, the people, what was going on and what used to happen. Eu morava em Curitiba quando conheci a Ana — I was living in Curitiba when I met Ana.

How to form it

There is nothing new to form: you combine the two tenses you already know. A quick reminder of the eu · ele forms:

The typical pattern is imperfeito for the scene + perfeito for the event, often joined by quando: Tava chovendo quando eu saí — It was raining when I left.

When to use it

Use the scene/event test. Does the sentence answer "what happened next?" (event → perfeito) or "what was it like?" (scene → imperfeito)?

• Scene: time, weather, place, people, feelings → Era tarde e fazia frio.
• Event: something that happened and finished → Aí eu ouvi um barulho.
• In progress + interruption: Eu tava dormindo quando o telefone tocou.
• Habit vs one occasion: Eu ia pra praia todo fim de semana. Um dia, fui sozinha.
• A chain of events takes the perfeito all the way: Cheguei, tomei banho e dormi.
• Signal words help but don't decide: ontem, de repente, aí, um dia, dois anos atrás lean to the perfeito; sempre, antigamente, todo dia, enquanto lean to the imperfeito.

• Eu morava em Curitiba quando conheci a Ana.I was living in Curitiba when I met Ana.background + event
• Era tarde, não tinha ninguém na rua, aí eu ouvi um barulho.It was late, there was nobody in the street, and then I heard a noise.scene, then event
• Eu não sabia da festa. Soube hoje de manhã.I didn’t know about the party. I found out this morning.sabia / soube
• Ontem eu trabalhei das oito às seis.Yesterday I worked from eight to six.a bounded period

Contrasts and traps

Some verbs change meaning with the tense:

• sabia = knew / soube = found out: Eu não sabia. Soube ontem.
• conhecia = knew already / conheci = met: Eu já conhecia o Rio, mas conheci a Bia lá.
• queria = wanted / quis = tried, insisted: Ele quis pagar a conta. And não quis = refused.
• podia = could, was allowed / pôde = managed or had the chance on one occasion. In speech Brazilians prefer consegui: Não consegui dormir.
• tinha = had, there was (scene) / teve = got, there was (an event): Tinha muita gente. Teve um acidente.

Two more traps:

• A long action is not automatically imperfeito. Ontem eu trabalhei dez horas is one finished, bounded block → perfeito. Eu trabalhava dez horas por dia is a habit → imperfeito.
• Spoken stories push events forward with aí, daí or então: Aí ele chegou e…

If you speak Spanish

Good news: the choice works almost exactly as between the indefinido and the imperfecto. Vivía en Curitiba cuando conocí a Ana = Eu morava em Curitiba quando conheci a Ana. Even the meaning shifts match: supe = soube, conocí = conheci, quise = quis, no quise = não quis. If you speak Spain Spanish, the one change is that he hecho for today's events becomes the simple perfeito: Hoje acordei cedo.