Language Guide
Language Guide: Preterite
This verb conjugation site is a great review of the formation of the forms of the regular preterite forms as well as those with spelling changes, stem-changes, and irregular forms. All sections have links to verb conjugation practice.
Study Languages
Study Spanish: Preterite Part 4
Great introduction to verbs with spelling changes in the preterite tense. A quiz at the end of the lesson tests the student's mastery of these conjugations.