Preterite vs Imperfect

• The preterite is used to express single actions in the past.
• Ayer, llegué tarde al trabajo.
  • Yesterday, I was late for work.
• The imperfect is used to show repeated actions.
• Llegaba tarde al trabajo a menudo.
  • I was often late for work.

-- We use the preterite when we refer to the action as a completed event even if it was a continuous or repetitive action:

• Aquella profesora enseñó biología el año pasado, pero nunca más.
  • That teacher taught biology last year, but not anymore.
• When we refer to the action when it was in progress, and not as a finished completed event, we use the imperfect:
• Aquella profesora enseñaba biología cuando yo aprobé el test.
  • That teacher was teaching biology when I passed the test.

• The preterite is used to express that an action is unique or an exception.
• The imperfect is used to express that an action is normal.
• Ella amaba jugar al fútbol pero aquel lo odió.
  • She loved to play football but that day she hated it.

• When two actions are taking place simultaneously, the preterite is used for the main action and the imperfect is used for ongoing actions.
• Mi madre cocinaba cuando la llamé por teléfono.
• My mother was cooking when I called her on the phone.