Preterite is usually for completed actions done in the past like played or began. It can only happen once. Typical phrases that let you know its preterite are ayer, una vez, anoche, la semana pasada.
Where as imperfect is ongoing or continuous actions. They typically have used to in front of the verb or the verb ending in -ing in english such as "He was speaking. (El hablaba.) Typical phrases in this tense include siempre, generalmente, toda la semana.