In Spanish, how do we know whether to use the preterite tense or the imperfect tense?

To start with, the preterite is used in two main cases: is the tense which you use to say (1)what you did or (2) what happened. To form the preterite of regular verbs, remove the –ar, –er or –ir from the infinitive, and add the following endings: (I would show a chart with all the different endings for three sample verbs).
On the other hand, the imperfect will be used in three different circumstances: (1)to say what was going on when something happened, (2)to give background description/set the scene, and (3)to say what you used to do or how things used to be. Just as with the preterite, To form the preterite of regular verbs, remove the –ar, –er or –ir from the infinitive, and add the following endings: (I would show a chart with all the different endings for three sample verbs).
(Along with the explanation, I would give examples of sentences in Spanish with verbs in the preterite/imperfect form that fit within each of these circumstances.)