This tense is for the ongoing past, and non specific. Like, When I was young, or used to doing things. I used to go fishing, I used to read all the time, I used to sleep late.   I was sleeping.  He was reading. 

I like this tense because it can sound funny.  A friend and I were exchanging funny-sounding words in Spanish and French.  My word was trabajaba.  Say it "trah-bah-ha-ba."

Ending: AR

ER/IR

I used to eat all the time.  Yo comía todo el tiempo.

When you were young you were happy. Cuando tú estabas joven, estabas contento.

Dormía.  I was sleeping.

Ser is irregular

Era        Eramos

Eras

Era        Eran

Ir  también

iba        íbamos

ibas      

iba        iban

Ver

veía        veíamos

veías

veía        veían

Imperfect Progressive is constructed by adding a past participle.   Past participle are formed by taking the ending off and adding ando for AR verbs and iendo for IR/ER verbs.  Comer=comiendo, hablar=hablando

¿Qué estabas haciendo?   What were you doing?

Estaba arreglando mi coche.   I was fixing my car.

Elena era muy seria.  No platicaba en clase. 

Elena was very serious.  She didn't chat in class.

Tú eras muy chistoso.  Siempre contabas buenos chistes.

You were(used to be) very funny. (Given to telling jokes)  You always told (were telling) good jokes.

Ustedes eran peresozos.  No estudiaban mucho ni salían bien en los exámenes.

You all were lazy.  You guys didn't study nor did you guys do well on tests.

Los veranos, íbamos al campo. 

We went to the county during the summer.

Cuando Ana y Paco eran novios, se hablaban todos los días.

When Ana and Paco were boyfriend/girlfriend, they spoke to each other every day.

Ayer a las cuatro, jugaba al tenís.

Yesterday at four, we were playing tennis.