1. Past progressive
The past progressive (pasado progresivo) is used to describe past actions in progress. It is formed with the imperfect of estar + the present participle of the main verb.
Nosotros estábamos hablando.

2. Past progressive
When the past progressive and the preterite are used in the same sentence, the past progressive describes an action in progress, and the preterite describes completed actions or interrupting events within that setting.

3. Past progressive Estábamos comiendo cuando llegó Cristina.
action in progress
interrupting event
Estábamos comiendo cuando llegó Cristina.

4. Past progressive
As with the present progressive, the past progressive refers to actions in progress.
Estaba lloviendo cuando salí de casa.

5. Past progressive
Past actions, conditions, or states that were ongoing or habitual, rather than in progress, referred to with the imperfect.
Llovía mucho en el pueblo donde vivían mis abuelos.