Preterite: spelling changes and -ir stem changes

Busqué mis llaves por todas partes. Al final, mi hermano las encontró.I looked for my keys everywhere. In the end, my brother found them.

Most regular preterite verbs just add the endings you'd expect. A handful of them, though, need a small spelling tweak or a stem change so the word keeps sounding — and being written — the way Spanish spelling rules require. None of this is irregularity in the deep sense: the endings are exactly the regular ones, only one letter or vowel shifts.

Quick shortcut: -car/-gar/-zar verbs change a letter only in the yo form; regular -er/-ir verbs with a vowel before the ending change unstressed i to y in usted/él/ella and ustedes/ellos/ellas; some -ir verbs weaken e→i or o→u in those same two forms.

-car, -gar, -zar: a spelling change in the yo form

Spanish spelling ties certain letters to certain sounds, and -é would break that link if the consonant stayed as written. So only the yo form changes:

Busqué las llaves. (I looked for the keys.) · Llegué a las ocho. (I arrived at eight.) · Empecé a trabajar temprano. (I started working early.)

Every other form of these verbs is completely regular: buscaste, buscó, buscamos, buscasteis, buscaron.

-er / -ir verbs with a vowel right before the ending: i → y

When the stem of an -er or -ir verb ends in a vowel (leer, oír, creer, construir...), the unstressed -i- of -ió and -ieron turns into -y-, because Spanish avoids writing an unstressed i between two other vowels:

Ella leyó el periódico. (She read the newspaper.) Ellos no me oyeron. (They didn't hear me.) The other forms stay regular, just with an accent to keep the stress in place: leí, leíste, leímos, leísteis.

This spelling change only applies where the rest of the verb's preterite is otherwise regular. Traer (to bring) also has a vowel before the ending, but its preterite is genuinely irregular in a different way — trajo, trajeron, never trayó/trayeron. Irregular preterites like this one are their own topic.

-ir stem-changing verbs: e → i and o → u

Some -ir verbs that change their stem vowel in the present tense (pedir → pido, dormir → duermo) carry a different, smaller vowel change into the preterite — but only in the usted/él/ella and ustedes/ellos/ellas forms:

Ella pidió un café. (She asked for a coffee.) Los niños durmieron toda la noche. (The children slept all night.) Every other form is regular: pedí, pediste, pedimos, pedisteis.

-ar and -er stem-changing verbs (pensar → pienso, volver → vuelvo) don't carry any stem change into the preterite at all — pensé, pensaste, pensó, pensamos... is completely regular. This vowel shift in the preterite is a feature of -ir verbs only.

Common mistakes

• ❌ Yo busque las llaves. → ✅ Yo busqué las llaves. (the qu spelling change is required before -é, and the accent stays)
• ❌ Ella leió el libro. → ✅ Ella leyó el libro. (an unstressed i between vowels becomes y)
• ❌ Ellos pedieron ayuda. → ✅ Ellos pidieron ayuda. (-ir stem-changers shift e→i in this form — the regular stem ped- doesn't appear here)
• ❌ Él piensó ayer. → ✅ Él pensó ayer. (pensar is a regular -ar verb in the preterite — the present-tense stem change pienso doesn't carry over)

Quick check

Here are five sentences to complete with the preterite form of the verb in parentheses.

1. Ayer yo ___ (llegar) tarde a clase.
2. Ella ___ (pedir) más tiempo.
3. Los estudiantes ___ (leer) el artículo.
4. Yo ___ (empezar) la tarea por la mañana.
5. El bebé ___ (dormir) toda la tarde.

Key takeaways

• -car/-gar/-zar verbs change a letter only in the yo form: busqué, llegué, empecé.
• -er/-ir verbs with a vowel before the ending change unstressed i to y in usted/él/ella and ustedes/ellos/ellas: leyó, leyeron.
• Some -ir verbs shift e→i or o→u in those same two forms: pidió, durmieron.
• -ar and -er stem-changing verbs don't carry any change into the preterite — it's completely regular for them.
• Some very common verbs (ser, ir, tener, hacer...) are genuinely irregular in the preterite, beyond a spelling or stem tweak — that's its own topic.