Llovía cuando ellos salieron. (It was raining when they got out.)
Llovía - imperfect tense because it describes the past situation
Salieron - past simple tense because it is a finished action in the past
Cuando llegué a la tienda ya estaba cerrada. (When I came the shop was already closed.)
Estaba - imperfect tense because it describes the past situation
Llegué - past simple tense because it is a finished action in the past
De repende me acordé de que tenía deberes. (I remembered that I had homework to do.)
Tenía - imperfect tense because it describes the past situation
Me acordé - past simple tense because it is a finished action in the past
Normalmente yo limpiaba mientras mi madre cocinaba. (Usually, I used to clean while my madre used to cook.)
Limpiaba, cocinaba - imperfect tense because these are two habits repetead regularly in the past (normalmente = usually)
(Normalmente yo limpio mientras mi madre cocina. - we could use the present tense in this case too)
Cuando Andrea corría por el parque los fines de semana, siempre veía a mis amigos. (When Andrea used to run in the park on weekends, she always used to see my friends.)
Corría, veía - mperfect tense because these are two habits repetead regularly in the past (siempre = always)
(Cuando Andrea corre por el parque los fines de semana, siempre ve a mis amigos. - we could use the present tense in this case too)