The Imperfect / El imperfecto

 A. REGULAR AND IRREGULAR IMPERFECT FORMS

1. Regular:
ayudar  ayudaba, ayudabas, ayudaba, ayudábamos, ayudabais, ayudaban (used to help)
atender  atendía, atendías, atendía, atendíamos, atendíais, atendían (used to pay attention to)
asistir  asistía, asistías, asistía, asistíamos, asistíais, asistían (used to attend)

2. Only three verbs have irregular forms in the imperfect:

ir:

iba ibas iba íbamos ibais iban

ser:

era eras era éramos erais eran

ver:

veía veías veía veíamos veíais veían

 B. USES OF THE IMPERFECT / USOS DEL IMPERFECTO

1. To describe in the past: the background or setting, situations, conditions, and actions that were in progress (was/were ...ing):

Eran las seis. Tenía doce años. Estaba enfermo y leía en cama. Afuera llovía. De pronto...

It was six. I was twelve. I was sick and was reading in bed. It was raining outside. Suddenly...

2. To describe habitual, customary actions, or things that used to happen in the past without specific reference to a beginning or end (imperfective, incomplete) (used to, would):

Iban al cine todos los viernes. Iba mucho al parque cuando era joven. Estaba informada porque leía mucho.

They used to go to the movies every Friday. I would to go the park a lot when I was young. She was (and stayed) informed because she (habitually) read a lot.

This means that time expressions conveying repetition are often clues for selecting the imperfect: con frecuencia, cada semana, siempre, a veces, etc.

Tip:  Whenever you could say used to or was/were...ing in English, you need the imperfect in Spanish.

The original website of this content + some exercises here.