what is it?
Some verbs change their spelling in the preterite, but only in the third person — él/ella and ellos/ellas. There are two patterns: a stem-vowel shift (e→i or o→u) in some -ir verbs, and a change of the written i to y when it falls between two vowels. The other persons stay regular.
when to use it
e→i and o→u in -ir verbs. Verbs like pedir, dormir, sentir and seguir shift their stem vowel only in the third person: pidió, durmió, sintió, siguió.
i→y between vowels. In -er and -ir verbs whose stem ends in a vowel (leer, oír, creer), the ending i becomes y: leyó, oyó, creyó.
Only the third person. The forms for yo, tú and nosotros keep the regular stem: pedí, dormiste, leímos.
formation
Start from the regular preterite endings, then apply the change in the third person only. The table shows both patterns side by side.
| person | pedir (e→i) | dormir (o→u) | leer (i→y) |
|---|---|---|---|
| yo | pedí | dormí | leí |
| tú | pediste | dormiste | leíste |
| él/ella | pidió | durmió | leyó |
| nosotros | pedimos | dormimos | leímos |
| vosotros | pedisteis | dormisteis | leísteis |
| ellos/ellas | pidieron | durmieron | leyeron |
Notice that the change appears only in the él/ella and ellos/ellas rows. Every other row uses the plain stem.
examples in context
common pitfalls
Keeping the infinitive vowel in the third person. Not pedió or dormió, but pidió and durmió. The stem vowel must shift.
Writing i instead of y between vowels. Not leió or oió, but leyó and oyó. An i trapped between two vowels is written y.
Applying the change to other persons. Not pidí or durmí, but the regular pedí and dormí. Only the third person changes.