1. Uses of the imperfect
The imperfect is used for actions that are not countable.
Used to do or used to happen (Habitual actions)
Yo iba al consultorio cada año.
I used to go to the doctor’s office each year.
Was/Were verb+ing (On-going actions in progress)
Yo jugaba cuando me lastimé.
I was playing when I hurt myself.

2. The imperfect is also used to express non-actions. Non-actions include:
Descriptions/Circumstances
Existence
Descriptions, circumstances, and existence are not actions. They do not express what happened, what someone did, what was happening, what someone was doing, what someone used to do, or what used to happen.

3. Había un chico en el parque. Había dos chicos en el parque.
The imperfect is used to express existence, or in other words, what used to be. (can’t count the times of being)
The imperfect form of hay is había (there was; there were; there used to be).
Like hay, había can be followed by a singular or plural noun.
Había un chico en el parque.
There was one boy in the park.
Había dos chicos en el parque.
There were two boys in the park.

4. Descriptions and circumstances express how a thing was/things were or what a person was like /people were like. (can’t count the times of a non-action)
El parque estaba en la ciudad.
The park was in the city.
Mi abuelo era médico.
My grandfather was a doctor.
Mis zapatos eran blancos.
My shoes were white.

5. Uses of the imperfect Era la una y media. (It was 1:30.)
Time-telling
(ALWAYS IMPERFECT)
Era la una y media.
(It was 1:30.)
Eran las tres menos cuarto.
(It was 2:45)
Age-telling
Los niños tenían seis años.
The children were six years old.
Name-telling
Mi hermano se llamaba Pablo.
My brother’s name was Paul.

6. When the day, month, year, date, or season expresses circumstance
(background to the main action) rather than reporting a fact, the
Imperfect is used instead of the preterit.
Telling the day Era lunes. It was Monday.
Telling the month Era abril. It was April.
Telling the year Era el año It was the year 2006.
Telling the date Era el 5 de mayo de It was May 5,2006.
Telling the season Era la primavera. It was spring.
IN CONTRAST: To report a fact, the preterit is used:
Ayer fue lunes.
Yesterday was Monday.

7. Uses of the imperfect Me enojaba. I was (feeling) angry.
Mental or emotional states
(feelings, beliefs)
(To express reactions,
which can be counted,
use the preterit)
Me enojaba. I was (feeling) angry.
(state/not countable)
Me enojé ayer. I was (got) angry yesterday. (reaction/countable)

8. To describe the weather Llovía cuando salimos.
(When it refers to what the weather It was raining when we left.
was like as background information Fui a la playa porque hacía
to the main action use imperfect.) sol.
I went to the beach because it
was sunny.
To report what weather Llovió ayer.
happened, use preterit. It rained yesterday.
Hizo sol el lunes pasado.
It was sunny last Monday.

9. Mi abuelo fue alto. My grandfather was tall.
Physical characteristics El médico era alto y guapo.
(to describe from the point of The doctor was tall and handsome.
view of the past; as though you
are mentally there in the past
describing)
Ella tenía el pelo rubio.
She had blond hair.
El chico llevaba pantalones cortos.
The boy was wearing shorts.
Mis zapatos eran de cuero.
My shoes were leather.
NOTE: If you are reporting a physical characteristic as a fact from the present point of view rather than describing it from the past point of view, use the preterit. This is considered a summary of actions or circumstances viewed as a whole and completed, rather than description.
Mi abuelo fue alto My grandfather was tall.
El chico llevó pantalones cortos ayer. The boy wore shorts yesterday.

10. Preterit for:
Action (countable number of times, including one time implied) This includes the beginning or the end of an action. (empezar a; comenzar a; teminar de; dejar de)
Reaction (countable number of times, including one time implied)
Summary statement (series of actions or circumstances viewed as a whole and completed)

11. Imperfect for: Habitual actions (can’t be counted=used to)
On-going actions in progress (can’t be counted = was/were doing)
Descriptions, circumstances, and existence
* Ir+ a + infinitive in the past always uses the imperfect.
Iban a estudiar. They were going to study.

12. RULE OF THUMB FOR VERBS IN THE PAST TENSE IN SPANISH
CAN’T COUNT THE TIMES = IMPERFECT
CAN COUNT THE TIMES = PRETERIT

13. To narrate a story in the past tense, both preterit and imperfect are needed.
You will see the preterit and the imperfect together in narratives such as fiction, news, and retelling of events.

14. In these cases the imperfect provides all of the background information, such as time, weather, location, and description, while the preterit indicates the specific events that occurred to advance the plot. (main actions)

15. Also, you will see various combinations of preterit and imperfect used in a single sentence.
Por ejemplo:
Los invitados bailaron y cantaron. (Two completed actions.)
Mientras bailaban, los invitados cantaban. (Two on-going actions)
Los invitados bailaban cuando yo llegué. (An on-going action interrupted. )

16. If the imperfect is used for on-going action in the same sentence as the preterit, the preterit action is one that interrupts. The word “mientras” (while) often precedes an imperfect on-going action. The word “cuando” often precedes the preterit interruption.