When talking or writing in the past tense, when should the preterite and when should the imperfect be used?

In Spanish, there are two past tenses; the preterite and the imperfect.

The preterite tense is used to describe a definitive, one-time action that has taken place and ended. For example, 'yesterday we ate apples'; 'ayer comimos manzanas'.

The imperfect tense is used to describe an action that took place over a period of time. In English, if the phrase 'used to' can be used to describe what is being said, the imperfect is used. For example, 'I used to go to school'; 'Yo iba a la escuela'.