Cambio vocálico: o>ue (poder, dormir, contar)

The o→ue stem change is the second most common pattern in Spanish, second only to e→ie. The mechanics are identical: a stressed o in the stem diphthongizes to ue; when the stress moves off the stem, the o stays. Poder gives puedo, puedes, puede but podemos, podéis, pueden. The pattern repeats across dozens of high-frequency verbs — dormir, contar, encontrar, volver, recordar — that you cannot avoid in everyday conversation.

For peninsular Spanish, the vosotros form is again the cleanest diagnostic: podéis, dormís, contáis — never puedéis, duermís, cuentáis.

The mechanism: stressed o → ue

Spanish inherited the o→ue pattern from Latin in the same way it inherited e→ie. The short Latin ŏ under stress regularly diphthongized to uo in early Castilian, then to ue in the modern language. The unstressed ŏ stayed as a simple o. The Romance languages each handled this split differently: Italian kept uo (buono), French dropped it (bon), Spanish landed on ue (bueno).

In the verb system, the result is the boot pattern: the o of the stem turns into ue whenever stress lands on it, and stays as o otherwise.

• yo PUEdo — stress on the stem → ue.
• nosotros poDEmos — stress on the ending → plain o.

That is the entire rule.

Poder — full paradigm

The classic model verb for o→ue is poder ("to be able to, can"), one of the most common verbs in Spanish.

Why vosotros podéis is the peninsular tell

In Latin America, the question "Can you (plural) come?" is ¿Pueden venir? — using ustedes, which keeps the diphthong because it's a third-person form (inside the boot). So the diphthongized stem is what learners hear constantly.

In Spain, by contrast, the everyday plural-you form is vosotros: ¿Podéis venir? — no diphthong, plain o. Once you start spending time in Spain, the rhythm puedo, puedes, puede, podemos, podéis, pueden becomes second nature precisely because the podéis form punctures the diphthong every time you address two or more friends.

The error of saying puedéis (with the diphthong) marks a learner as unmistakably non-native within a single sentence.

Dormir — full paradigm

Dormir ("to sleep") is the model -ir o→ue verb. The stem changes in the present, and — because it's an -ir verb — it also extends the change into the preterite (3rd persons) and gerund, where the diphthong shifts to a plain u.

A preview of dormir beyond the present: the gerund is durmiendo (o→u, not o→ue), and the 3rd-person preterite is durmió, durmieron (also o→u). The same applies to morir (to die): muero in the present but muriendo, murió, murieron. This o→u shift is exclusive to -ir verbs in this family — poder (-er) doesn't do it. See o→u in the preterite for the full story.

Contar — full paradigm

Contar is a delightful verb because it means two things at once: "to count" (numbers) and "to tell" (a story). Both senses run through the same paradigm.

Note the idiom contar con + person ("to count on someone") — extremely common in everyday speech.

A working list of o→ue verbs

These are the high-frequency o→ue verbs across all three conjugation classes. All of them follow the same boot pattern.

-ar verbs

-er verbs

-ir verbs

The -ir list is short — just dormir and morir. But these two will appear again in the preterite and gerund with a further shift (o→u): durmió, muriendo.

A peninsular collocation note: ¿Cuánto cuesta?

The phrase ¿Cuánto cuesta? ("How much does it cost?") is the workhorse question in any shop, market, or taxi. The verb costar is one of those Spanish verbs that runs almost exclusively in the third person — like gustar and doler. You won't say yo cuesto in the literal sense (unless you're a very expensive person). The relevant forms in practice are cuesta (singular item) and cuestan (plural items).

The verb also has a metaphorical use, "to be hard for someone": Me cuesta entenderte ("It's hard for me to understand you"). Same paradigm.

Verbs in the gustar family

A subset of o→ue verbs work like gustar — the thing experienced is the grammatical subject, and the experiencer is an indirect object. The verb mostly shows up in third person.

• doler — Me duele la cabeza (my head hurts).
• costar — Le cuesta concentrarse (it's hard for him to concentrate).
• encontrar — used reflexively as encontrarse (to feel): Me encuentro mal (I feel sick).

These don't violate the boot — they just don't use the first/second persons much in normal life.

How English speakers go wrong

English has no equivalent of the o→ue alternation. The verb "I can" is can across every person. So learners pick up puedo, puede (the textbook forms) but then have to manually suppress the diphthong when conjugating nosotros and vosotros. The instinct to over-apply the diphthong is strong.

There's also a vocabulary confusion to watch out for:

• Pueder — does not exist. The infinitive is just poder.
• Conter — does not exist. The infinitive is contar.
• Duermir — does not exist. The infinitive is dormir.

The unstressed o of the infinitive is the true stem of the verb. The diphthongized ue is what surfaces under stress. So infinitives never carry the diphthong.

Common mistakes

This is the cardinal peninsular error in this family. The vosotros form drops the diphthong.

This is a different kind of mistake — confusing contar (to count, to tell) and costar (to cost). They look alike, but only one of them is the price verb.

Key takeaways

• The o→ue pattern affects -ar, -er, and -ir verbs. It's the second-most-common stem change after e→ie.
• The change applies in the four "boot" forms: yo, tú, él/ella/usted, ellos/ellas/ustedes.
• Nosotros and vosotros do not carry the diphthong: podemos, podéis, not puedemos, puedéis. This is the peninsular tell.
• Two -ir o→ue verbs (dormir, morir) extend their change into the preterite and gerund as o→u: durmió, durmiendo, murieron, muriendo.
• A few o→ue verbs (costar, doler, llover) work like gustar — used mostly in the third person, with the experiencer as an indirect object.
• The infinitive always shows the unstressed stem (with o, not ue). The diphthong only surfaces in stressed positions.