The preterite is used to narrate completed actions in the past. Certain words like ayer (æyesterdayÆ), anoche (ælast nightÆ), el año pasado (ælast yearÆ), and la semana pasada (ælast weekÆ), are associated with the preterite, since they usually refer to specific events that are now completed.

The imperfect is used to narrate actions that were habitual or ongoing in the past. There is no reference to whether or not these things were ever completed, or whether they continue into the present time. Phrases like siempre (æalwaysÆ), frecuentemente (æfrequentlyÆ), and todos los días (ædailyÆ), are often associated with the imperfect. The difference between the preterite and imperfect is often very subtle, and even native speakers will occasionally disagree about which form is correct. You should study examples and try to imitate native speakers when possible.