Preterito vs Imperfecto

Preterito vs Imperfecto, when describing events or situations in the past, Spanish speakers must choose between the preterite and the imperfect. The choice depends on the aspect of the event or situation that the speaker wants to describe. In theory, every action has three phases or aspects; a beginning, a middle and an end. When a speaker focuses on the beginning or the end of an action, the preterite is used. When he or she focuses on the middle, the imperfect is used.

Uses of the Imperfect tense vs preterite tense is one of the greatest problems for the Spanish student.
The Imperfect tense is much less commonly used in English than in Spanish. since the word Imperfect means not perfect or not completed, the imperfect
tense in used to express continuance, to express those actions in the past which are either customary or habitual. Some common adverbial expression which would indicate continuance and thus demand the use of the imperfect are:

Siempre: always
Con frecuencia: frequently
Frecuentemente: frequently
A menudo: often
A veces: sometimes
De vez en cuando: sometimes, from time to time
Muchas veces: many times
Cada dia: every day

We use Imperfect:

repeated action
Habitual action
indefinite period of time
Setting the scene
Weather
Time
Description of a scene
ongoing/Interrupted action
Personal description
Age
Emotions/ Feeling/ Beliefs
Status of mind / Body
Physical appearance

Uses of the Preterite tense

The preterite is often referred to as the past definite. It is used to express an action that was completed at a definite time in the past. Some common adverbial phrases that accompany the preterite are:

Ayer: yesterday
Anteayer: the day before yesterday
anoche: last night
el otro dia: the other day
hace dos dias: two days ago
la semana pasada: last week
el mes pasado: last month
durante tres siglos: for three centuries

Preterite express:

Events
One time event
Chain of event in a storyline
Actions within a defined period of time’
isolated action
Interrupting action