The preterite is an action that has already occurred, and It is something that has a definite finished time. It describes the action from the past, which is already done with and not happening anymore. It is also used when there is a specific and separate starting place and ending place. An example of this would be "at the time I felt depressed." This example already occurred.
Another example would be "the night before last; I slept like a baby." It describes something that already happened. The imperfect tense is used to speak about past actions that are lacking a beginning or an end. For example, "he was playing." Again, this is past tense, and the focus is carried in the action which is imperfect in this sentence. In the Spanish language, the verb is synonymous with imperfect tense.