We can use the preterite and imperfect together to talk about the past.

• Use the imperfect to say what was going on or what was happening in the background.
• Use the preterite when what was happening in the background was interrupted by another action.

In the following example, imperfect tense verbs are in ( ).

• Mientras (hablaba) sono' la alarma contra incendios.
  While I was talking the fire alarm went off.

• Baje' las escaleras y, al llegar a la calle, me di cuenta de que (hacía) mucho calor.
  I rushed down the stairs and, when I got to the street, I realized that it was very hot.