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?
¿Qué hacían ustedes cuando sonó la alarma de incendios?
What wre you doing when the fire alarm went off.
Nosotros cruzábamos la calle cuando vimos el camión.
We were crossing the street when we saw the truck.
Mi esposa y yo estábamos en la habitación del hotel cuando explotó la bomba.
My wife and I were in our hotel room when the bomb exploded.
¿Cuántos años tenías cuando Neil Armstrong llegó a la Luna?
How old were you when Neil Armstrong landed on the Moon?
B. We combine the imperfect with the imperfect to talk about simultaneous actions that were taking place in the past, using the conjunction mientras (while) to indicate simultaneity.
Sonia cocinaba mientras el bebé dormía.
Sonia was cooking while her baby was sleeping.
Antonio escuchaba la radio mientras limpiaba el cuarto de baño.
Antonio was listening to the radio while he was cleaning the bathroom.
El Because de los Beatles sonaba mientras me lavaba los dientes.
The Beatles’ Because was playing while I was brushing my teeth.
Bailábamos mientras la orquesta tocaba.
We were dancing while the orchestra was playing.
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?
¿Qué hacían ustedes cuando sonó la alarma de incendios?
What wre you doing when the fire alarm went off.
Nosotros cruzábamos la calle cuando vimos el camión.
We were crossing the street when we saw the truck.
Mi esposa y yo estábamos en la habitación del hotel cuando explotó la bomba.
My wife and I were in our hotel room when the bomb exploded.
¿Cuántos años tenías cuando Neil Armstrong llegó a la Luna?
How old were you when Neil Armstrong landed on the Moon?
B. We combine the imperfect with the imperfect to talk about simultaneous actions that were taking place in the past, using the conjunction mientras (while) to indicate simultaneity.
Sonia cocinaba mientras el bebé dormía.
Sonia was cooking while her baby was sleeping.
Antonio escuchaba la radio mientras limpiaba el cuarto de baño.
Antonio was listening to the radio while he was cleaning the bathroom.
El Because de los Beatles sonaba mientras me lavaba los dientes.
The Beatles’ Because was playing while I was brushing my teeth.
Bailábamos mientras la orquesta tocaba.
We were dancing while the orchestra was playing.