While textbooks have approached the troublesome practice of teaching the preterite and imperfect in a variety of ways, I have found that those that insist on a "Spanish-only" approach, working strictly in the target language, simply do not deliver the best results when it comes to student comprehension of the past tenses. A concept that does not exist in one's own native language (assuming here that it is English) can not be explained using the foreign language. The difference between preterite and imperfect seems to be one of the most difficult cases which is not best served by the modeling and example techniques afforded by the proficiency or natural methods, especially when practiced on older adolescents and adults who are more likely to analyze than to model.
Maury, Debra, "FLARR Pages #9: Teaching Preterite vs. Imperfect in Spanish" (1998). FLARR Pages. 37.