Before knowing how and when to use the preterite aspect, you must know how to conjugate "ar," "er" and "ir" verbs. "Ar" verbs take on the following endings -- replacing the infinitive "ar" ending -- when conjugated in the first-, second- and third-person singular and first-, second- and third-person plural respectively: é (yo), aste (tú), ó (él/ella/usted) and amos, asteis, aron. "Er" and "ir" verbs are conjugated í (yo), iste (tú), ió (él, ella, usted) and imos, isteis, ieron.
Identify whether the verb being used is referring to a limited time frame or period of time in the past. If the verb refers to a specific time frame or period of time when the action was committed in the past, use the preterite tense. For example: "When I was 6, my brother walked with me to school." You should apply the preterite verb conjugation to the verb walk because it specifically refers to "when I was 6."
Identify whether the verb is referring to a reaction or a change in condition in the past. While the imperfect aspect is generally used when referring to past conditions or beliefs, the preterite is used to depict changes in beliefs, conditions or feeling in the past. This particularly applies to reactions to an event. For example: "I liked the movie we saw last night." The verb "liked" should be conjugated in the preterite aspect, because it is referring to someone's reaction to an event.
Identify from what angle and to what point in time you are referring to an action taking place. The preterite tense should be used when referring to the beginning or end of an action or a completed action. For example: "I started work at 3." The verb started should be conjugated in the preterite aspect because it refers to the beginning of an action. On the other hand, the imperfect tense would be used if referring to the middle point, progress or interruption of an occurring action.
Also apply the preterite aspect conjugation when referring to a list of consecutive actions. You would use the preterite construct when compiling a list of completed actions, such as: "I woke up, showered, ate breakfast and left the house." Each of these verbs should be conjugated in the preterite aspect.