The imperfect tense (el pretérito imperfecto) in Spanish is used to communicate certain actions/events that occurred in the past without a specific timeline, duration and frequency. In the imperfect tense, the actions do not have a definite beginning or end.
Example:
Juana tenía dos gatos.
Juana had two cats.
In the example above there is no explanation of when or for how long Juana owned the cats.
The difference between the preterit (el pretérito) and the imperfect tense (el pretérito imperfecto) is that the imperfect tense does not refer to the specific time an action took place (the beginning or end is not mentioned), instead, it describes the duration of the action(s) and how frequently the action took place.
Example:
Cuando Juan vivía en España iba mucho a los museos.
When Juan lived in Spain, he used to go to the museums.
The imperfect tense can be used in the same sentence with the preterit tense.
Example:
Mi mamá cocinaba la cena cuando de repente sonó el timbre.
My mom was cooking dinner when suddenly the doorbell rang.
In the example above, the verb ‘cocinaba‘ is in the imperfect describing the action in the past, while, the sudden action of ringing the bell (sonó) is in the preterit tense. The preterit tense is the appropriate tense to express the “interrupting” action.
Example:
Lucas era un muy buen arquitecto.
Lucas was a very good architect.
Example:
Isabel compraba un café todas las tardes después del trabajo.
Isabel would buy (used to buy) a coffee every after afternoon after work.
Example:
Yo tenía veintisiéte años cuando me casé.
I was twenty seven years old when I got married.