How do I know when to use the preterite or the imperfect tense?

In general:·   
 Imperfect: tense to describe and explain actions in development·     
Preterite: tense to narrate and explain completed actions

Preterite:
1.To talk about a series of past actions: Me levanté, y después fui al cine
2. To talk about the beginning or the end of a past occurrence: El espectáculo comenzó en la mañana
3. To talk about things that are completed: Compraste el bocadillo ayer


The Imperfect:
1.To describe an action that a person did habitually in the past: Cuando era pequeña, iba al cine todos los sábados.
2.To describe an action without reference to any beginning or end: Juan tenía hambre constantemente.
3.To describe physical, emotional and mental states or conditions: Estaba tan alegre y quería bailar
4.To tell time Eran las seis de la tarde. Era la una de la mañana.
5.To describe an action or state of being that took place in the past and lasted for a certain length of time before to another past action.  Estaba en el salón, y comía un helado cuando mi hermano entró.