The most common past-tense verbs do not follow the regular endings and have to be learnt as forms. Ser and ir share exactly the same preterite (fui, foste, foi, fomos, foram) and only context tells them apart; fazer gives fiz/fez, ter gives tive/teve, estar gives estive/esteve, and poder gives pude/pôde.
Examples
- Ontem fui ao cinema.
- Yesterday I went to the cinema.
- Fiz o jantar às oito.
- I made dinner at eight.
- Tive um dia muito longo.
- I had a very long day.
The mistake to avoid
"Fui" is both "I went" and "I was" - the sentence around it decides which.
Where this fits in
Falar do passado, Portuguese A2: Contar o que fez ontem, no fim de semana passado, e as suas experiências de vida com o pretérito perfeito.