Uses of the Imperfect with the Preterit in the same sentence

A. Both ways of expressing the simple past in Spanish, the preterit and the imperfect, are combined when an action interrupts another action that was in progress in the past. That is to say, something happened when something else was happening in the past. The action that was taking place is expressed by the imperfect; the action that occurred later at a specific moment requires the use of the preterit.

La policía interrogaba al sospechoso cuando llegó el sargento.
The police was interrogating the suspect when the sergeant arrived.

¿Qué hacías cuando sentiste el temblor de tierra?
What were you doing when you felt the earth tremor?