The most important things to remember about stem-changing verbs in the Preterite Past Tense are that:
1) -Ar verbs that stem-change in the present tense (e.g., mostrar, almorzar, pensar and -Er Present tense stem-changing verbs (e.g., Perder, Tener, entender) do not stem-change in any other tenses including the Preterite.
2) -Ir verbs that stem-change in the present tense (e.g. Dormir, Servir, Preferir, etc.) continue to have stem-changes.
- However, these stem-changes occur only in the 3rd person singular (él, ella, usted) and 3rd person plural (ellos, ellas, ustedes).
- They follow the same stem-changing pattern they have in the present progressive tense ("-iendo"):
- The "O-->UE" stem-changers change to "U" as they did with the "-ing" (-iendo) form:
- Dormir: él está durmiendo, él durmió
- And the E--->IE and the E--->I change to "I" as they did with the "-ing" (-iendo) form:
- Servir: Ella está sirviendo cerveza, ella sirvió cerveza.
| Yo | tú | él, ella, usted | nosotros | ellos, ellas, ustedes | |
| Dormir (ue,u) |
dormí | dormiste | durmió | dormimos | durmieron |
| Morir (ue, u) |
morí | moriste | murió | morimos | murieron |
| Servir (i, i) |
serví | serviste | sirvió | servimos | sirvieron |
| Preferir (ie, i) |
preferí | preferiste | prefirió | preferimos | prefirieron |
| Repetir (i, i) |
repetí | repetiste | repitió | repetimos | repitieron |
Pedir |
pedí | pediste | pidió | pedimos | pidieron |
When you see stem-changing verbs listed in a text book or in a dictionary with vowels in a parenthesis, they are followed by the stem-change in the present tense in parenthesis: Pensar (ie), Entender (ie), Almorzar (ue).
Ir stem-changing verbs have two sets of vowel changes separated by a comma. This indicates first the stem-change in the Present Tense and then the stem-change for all other tenses: Preferir (ie, i) él prefiere (present), él prefirió (preterite)