Normally, there is a lot of confusion between when you have to use Pretérito Imperfecto or Pretérito Indefinido (in some books this appears as “Pretérito Perfecto Simple”.)
The “Pretérito Indefinido” tense, is a tense that is used when we are speaking about things that happened one time in the past, Ex: “Esta mañana desayuné frutas” or ...