There are various ways in Spanish to express an action in the past. This exercise treats the simple past, i.e. a verb that is a one-word verb, is already completed, and took place only once. Frequently the past tense is also called the preterit. Again, the action is complete and happened only once. Please refer to earlier sections in order to review regular AR,ER,IR and stem change preterits.
Irregular preterits are irregular because of the way they evolved from Latin.
This being said, the best way to learn them is pure memorization, starting with their irregular stem. You have already gotten in the habit of dropping the last two letters and adding a news set of endings. SER and IR have identical irregular preterits. Di, diste, dio. In the exercise that follows, match the subject and verb with the irregular preterit.