Pretérito Imperfecto vs Pretérito Indefinido.

Imperfect Tense 

Actions in progress at a specific moment in the past: The imperfect is used to talk about actions that were occurring at a given time and we are not concerned whether they were completed or not.

• Example: “Cuando era niño, jugaba en el parque todos los días.”

• Example: “Leía tranquilamente cuando de repente sonó el teléfono.”

Interrupted actions: We use the imperfect to describe an action that was interrupted by another action, which is usually in the preterite.

• Example: “Estaba cocinando cuando llegó mi madre.”

• Example: “Veíamos una película cuando se fue la luz.”

Descriptions: The imperfect is ideal for describing people, situations, or contexts in the past.

• Example: “El cielo estaba nublado y hacía frío.”

• Example: “Mi abuela era alta y siempre sonreía.”

Habits or past customs: It is used to talk about actions that were habitually repeated.

• Example: “Antes caminaba a la escuela todos los días.”

• Example: “Cada verano íbamos a la playa.”
  

Preterite Tense 

Completed actions in the past: It is used to talk about actions that have a clear beginning and end in the past.

• Example: “Ayer terminé mi libro.”

• Example: “Compramos nuestra casa en 2005.”

Narration of events: Ideal for recounting specific events or actions that occurred in the past.

• Example: “El concierto empezó a las ocho y terminó a las diez.”

• Example: “Columbus llegó a América en 1492.”
  

Comparisons between Imperfect and Preterite 

Repeated actions: The imperfect is used for repetitive actions without a defined number of times; the preterite, for repeated actions with a clear limit.

• Imperfect: “De niño, visitaba a mis abuelos cada domingo.”

• Preterite: “De niño, visité a mis abuelos cada domingo durante tres años.”

Narrations: In narratives, the imperfect describes the setting or context, while the preterite moves the action forward.

• Imperfect: “Era una noche oscura y tormentosa.”

• Preterite: “De repente, un relámpago iluminó el cielo.”

Two actions in the past: The imperfect allows us to see the action from within as a process; the preterite, as a concluded event.

• Imperfect: “Bebíamos mucho durante la fiesta.”

• Preterite: “Bebimos mucho durante la fiesta.