Verbs that have stem changes in the present tense do not have the same change in the preterite. Verbs ending in -ar or -er have no stem change in the preterite: pensar (ie) > pensé, pensaste, pensó, pensamos, etc.; volver (ie) > volví, volviste, volvió, volvimos, etc.

Verbs ending in -ir do have a stem change in the preterite, but only in the third person singular and plural. This change is not always the same change as for the present tense. In vocabulary lists, stem changes for the preterite are listed in parentheses next to the stem change information for the present tense.

DORMIR (ue,u)

dormí, dormiste, durmió, dormimos, dormisteis, durmieron

PEDIR (i,i)

pedí, pediste, pidió, pedimos, pedisteis, pidieron

Verbs ending in -car, -gar, and -zar have a spelling change in the first person singular of the verb, where c > qu, g > gu, and z > c. Recall that a similar spelling change occurs for these verbs in the formal commands and in the present subjunctive.
 

buscar > busqué

pagar > pagué

empezar > empecé

Verbs ending in two unstressed vowels have a spelling change in the third person singular and plural:

creer > creyó, creyeron

leer > leyó, leyeron

caer > cayó, cayeron