If you haven't watched this video yet (Past tense regulars) you have to watch this explanation first and click this link https://lalcenter.com/spanish-past-tense-regulars/. After you watch that video this one will be more clear to you. Here are all the materials as I promised in the video. While you watch it you can take notes, and follow the materials in this link https://lalcenter.com/spanish-past-stem-changing-verbs-irregulars/