The imperfect is a form of the past tense. We use it when:
- We talk about things that happened an indefinite amount of times (if I know the number of times it's preterit and not imperfect) (When I was (era) little...)
-We talk about things that were happening at the same time (I was talking (hablaba) on the phone when he came in)
-Any time I use "ing" in the past (I was walking (caminaba) to your house when...)
-Any time I want to say "used to" (I used to go (iba) to summer camp every summer)
-Any time we talk about a habit (I washed (lavaba) my car every Sunday)
-Any mental and emotional actions such as wanted, loved, needed, felt, etc.
-For weather, time, date, and age (When I was 9/cuando tenía 9 años. Or It was January 15/Era el 15 de Enero)

Formation:
Endings for Ar: aba, abas, aba, ábamos, abais, aban
Endings for Er/ir: ía, ías, ía, íamos, íais, ían

Hablar: hablaba, hablabas, hablaba, hablábamos, hablabais, hablaban
Comer: comía, comías, comía, comíamos, comíais, comían

Irregulars: (there are only 3!)
Ser: era, eras, era, éramos, erais, eran
Ir: iba, ibas, iba, íbamos, ibais, iban
Ver: veía, veías, veía, veíamos, veíais, veían