Telling a Story
For my spanish class we have to do a retelling of a legend of folktale so I'm doing goldilocks and the three bears. I don't know what to do for the tense though! Should I tell the whole thing in the imperfect? When (if I should) do I switch from imperfect to the preterite?
Much appreciation for your help! I've been pouring over my spanish book and it's zero help 
3 Answers
I assume you want to tell the entire story in the past. You should use the imperfect for all descriptions in the story. ex. "Hacía mucho sol" or "El oso era pequeño." You should use preterite for all action in the story. For example, "abrió la puerta." However, if something was happening when something else interrupted it, you will need both preterite and imperfect. ex. "Andaban cuando él dijo..." Spanish teachers love to see the use of as many different tenses as possible, so I would suggest using estar(imperfect) + past participle. To use the example above, "Estaban andando cuando..." This is not necessary, but should be helpful. If you have any more questions, I would be happy to help.
Once upon a time- Había una vez
There are very good reference tools on this website for differences between imperfect and preterite. One thing to remember (hopefully this won't confuse you too much), use the imperfect if it's a description in the past, ej. she was blonde- era rubia. If the action has a definite end in the past, then you would use preterite, ej.she finished eating dinner- terminó cenar. You should find the preterite and imperfect key words very helpful.
MelissaR did an outstanding job explaining the use of the preterite and imperfect, and this should probably cover about 90% of the story, so I really don't have much to add except one thing. Just remember that when the story shifts to a section with dialog the tense often shifts to some form of the present...."Who has been sleeping in my bed", or "Who has been eating my porridge," etc. [progressive form of the present perfect - ha estado comiendo/durmiendo].
