How do you know when you need to use the preterite or the imperfect when talking about past events?

The preterite is used when talking about completed actions or events in the past. e.g. Last week I wrote to my Spanish friends. / La semana pasada escribí a mis amigos españoles.e.g. Yesterday my friend Pedro and I went to the swimming pool. / Ayer pedro y yo fuimos a la piscina. The imperfect is used to talk about:- Past actions or events where the beginning or the end of the action is not clearly defined. e.g. Juan was playing tennis / Juan jugaba al tenis.- Repeated actions in the paste.g. When I was younger, I used to live in the city. / Cuando era joven, vivía en la ciudad. - Descriptions in the past rather than events (i.e. when setting the scene in a story)e.g. The man was short and was wearing a green jacket. / el hombre era bajo y llevaba una chaqueta verde.- Time in the paste.g. It was 1 o'clock / Era la una.