Preterite stem changes for -ir verbs

Some -ir verbs that have a stem change in the present tense also have a stem change in the preterite (past) tense. However, in the preterite, this change only happens in the third-person forms: singular (él, ella, usted) and plural (ellos, ellas, ustedes).

The "e" to "i" change

For verbs like pedir (to ask for), preferir (to prefer), sugerir (to suggest), and seguir (to follow), the e in the stem changes to an i in the third person.

PronounPreterite of "pedir"
yopedí
pediste
él / ella / ustedpidió
nosotros / nosotraspedimos
ellos / ellas / ustedespidieron

Notice how the change only affects the bottom rows of the conjugation chart:

It is ten o'clock and the guest asked for another blanket.
Son las diez y el huésped pidió otra manta.
They preferred that soap.
Ellos prefirieron ese jabón.
The man suggested buying the cheap shampoo.
El hombre sugirió comprar el champú barato.
The guests followed the manager through the hallway.
Los huéspedes siguieron al gerente por el pasillo.

The "o" to "u" change

For the verb dormir (to sleep), the o in the stem changes to a u, following the exact same third-person rule.

PronounPreterite of "dormir"
yodormí
dormiste
él / ella / usteddurmió
nosotros / nosotrasdormimos
ellos / ellas / ustedesdurmieron
She did not sleep because the elevator made a lot of noise.
Ella no durmió porque el ascensor hizo mucho ruido.
The guests did not sleep well yesterday.
Los huéspedes no durmieron bien ayer.