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

A: In general, you use the preterite tense to describe completed actions in the past, these are often single actions that took place at a specific moment in time. For example:El año pasado fui a Barcelona. Last year I went to Barcelona.Ayer Juan y Miguel jugaron fútbol a la playa. Yesterday Juan and Miguel played football on the beach.Hace dos semanas Ana envió una postal a su amiga. Two weeks ago Ana sent a postcard to her friend.In all of these instances there are clear markers of time (last year; yesterday; two weeks ago) which show it is a one-time event that is now completed.We usually use the imperfect tense to describe repeated, usual or habitual actions in the past. For example:Cada verano iba a Alicante con mi familia. Every summer I went to Alicante with my family.Luis siempre comía papas a la casa de su abuela. Luis always used to eat chips at his Grandmother’s house.Usualmente viajaban en avión. They usually travelled by plane.In these examples the key words show that the action has been repeated (every summer; always; usually). There is no clear beginning or end to these actions which is why we use the imperfect tense.