When should I use the imperfect and preterite past tenses in Spanish?

Good question. This can be confusing because we don't always have the same markers for these two tenses in English. Try to remember the preteriteas one single action or time period, and the imperfectas either something ongoing, or something that is repeated.The imperfect is often used to SET THE SCENE. That's why you normally see imperfect used with descriptions of places and weather in the past. For example: "Hacíasol en el jardín." After the scene is set, the main action that occurs next will usually be in the preterite: "Hacíasol en el jardín cuando Daniel llegó."In the same way, if one action is happening continuously and is then interrupted by a second, more sudden action, the first will be in the imperfect(because it is ongoing and it is setting the scene for the next action). The second verb will be in the preterite(because it is one single action that happen once). For example: "Mi madre eraleyendo el periodico cuando mi padre se cayó."The imperfectis also used for CONTINUOUS or REPEATED actions in the past. For example, if you are talking about your holiday and want to say that during a week-long holiday, you swam every day, use theimperfect. For example: "Nadábamosen la piscina todos los días." However, to mention something you did on just one occasion, use the preterite. "Nadábamosen la piscina todos los días, pero el miercoles tambien yo nadé en el mar."