El indicativo del imperfecto / Imperfect Indicative

Imperfect indicative tense is used to express a repeated action in the past or an interrupted action in the past. Examples in English:

• I used to play soccer. [repeated]
• I was sleeping when it happened. [interrupted; referring to first verb]

All you do to form it is take the infinitive form of the verb, take off the -ar/-er/-ir and add the following endings:

For -ar verbs:

• yo: -aba
  
• tú: -abas
• él/ella/Usted: -aba
• nosotros: -ábamos
• (vosotros: -abais)
• ellos/ellas/Ustedes: -aban

For -er and -ir verbs:

• yo: -ía
• tú: -ías
• él/ella: -ía
• nosotros: -íamos
• (vosotros: íais)
• ellos/ellas/Ustedes: -ían
  

Here's how it can be translated (sometimes it depends on context):

• I/you/he/she/it/we/they were [verb]ing
• I/you/he/she/it/we/they used to [verb]
• I/you/he/she/it/we/they [verb]ed

There are a few others, but they are very rare. These three are the most common, especially the first one. Do not confuse the last one with the preterite. Although they are sometimes translated the same into English, they mean two different things.

Here are two examples of verbs conjugated in the imperfect, one -ar and one -er:

sacar (to take out)
yo sacaba=I was taking out
tú sacabas=you were taking out
él/ella/Usted sacaba=he/she/you were taking out
nosotros sacábamos=we were taking out
(vosotros sacabais=you were taking out)
ellos/ellas/Ustedes sacaban=they/you were taking out

comer (to eat)
yo comía=I was eating
tú comías=you were eating
él/ella/Usted comía=he/she/you were eating
nosotros comíamos=we were eating
(vosotros comíais=you were eating)
ellos/ellas/Ustedes comían=they/you were eating