1. El pretérito y el imperfecto
Como y cuando usar los tiempos pasados en español

2. El pretérito
Generally speaking, the preterit is used for actions in the past that are seen as completed. Use of the preterit tense implies that the past action had a definite beginning and definite end.
For example:
Compré uno zapatos para mi hijo. I bought a pair of shoes for my son.
¿Dónde comió usted ayer? Where did you eat yesterday?
Comí en casa de Carlos y hablé con su padre. I ate at Charles’s house and spoke with his father.

3. El pretérito
The preterit is used for past events that happened once and at a very specific moment in time, such as “Last night I went to the movies.”
The preterit is also used for incipient action, meaning action that we see begin, such as "the sun came out" as opposed to an imperfect weather statement such as "It was sunny yesterday".
When action begins or ends (ex: “I met Felipe. She graduated from college. The class ended at 9:15.”) we use the preterit.
Furthermore, when action is interrupted (ex: “We were golfing when it started to rain.") the verb that stops the action is in the preterit.

4. El pretérito The conjugations are as follows: AR -é -amos
-aste -asteis
-ó -aron
ER/IR
-í -imos
-iste -isteis
-ió -ieron

5. El imperfecto
The Spanish imperfect usually translates into English as "used to" or "would" for example:
When we were young we USED TO EAT cereal. (We may still eat cereal today.)
As a child we WOULD EAT cereal. (We might still.)
In the sentence "As a child I lived in Chicago" the listener does not have enough information to determine when the speaker arrived in Chicago or when he/she left. It is possible that he/she still lives there.
The imperfect can also be used as a past progressive, such as "They WERE EATING".
The general rule is that if you do not know when the action began or ended, it takes the imperfect.
However, there are some cases in the past that almost always take the imperfect.
Unless you have a clear beginning (incipient action), use the imperfect to describe the following in the past:

6. El imperfecto Age Tenía 12 años cuando robé el primer carro. Weather
I was 12 when I stole my first car.
Weather
Hacía buen tiempo cuando me arrestaron.
It was nice weather when they arrested me.
Emotional states
Mi amigo estaba nervioso porque era la medianoche.
My friend was nervous because it was the middle of the night.
What people were doing WHEN something else happened
Me duchaba cuando sonó el teléfono.
I was showering when the phone
rang.
Habitual past actions
Íbamos a la playa cada verano.
We went [used to go] to the beach
every summer.
Time
Eran las nueve de la noche.
It was nine o'clock at night.

7. El imperfecto Words that are hints for using Imperfect: Siempre
Cada (año, día...)                        
Usualmente
Generalmente
Normalmente
En general
En el pasado...

8. El imperfecto The conjugations are as follows: AR -aba -abamos
-abas -abáis
-aba -aban
ER/IR
-ía -íamos
-ías -íais
-ía -ían

9. Solo hay tres verbos irregulares en el imperfecto …
~BUENAS NOTICIAS~
Solo hay tres verbos irregulares en el imperfecto …
SER – era, eras, era, éramos, erais, eran
IR – iba, ibas, iba, íbamos, ibais, iban
VER – veía, veías, veía, veíamos, veíais, veían