When do you use the imperfect and when do you use the preterite tense?

Imperfect is used if you have done an event in the past over a period more than once. Preterite is used when the event has been and already finished in the past. Jugaba deporte en mi colegio - I played sport at school (not one single time) Comi la comida rapidamente - You ate the food quickly! (one single time when you ate the food quickly)