Duermo ocho horas. Prefiero dormir más.I sleep eight hours. I prefer to sleep more.

Some very common Spanish verbs change a vowel in their stem when they are conjugated: dormir becomes duermo, querer becomes quiero, pedir becomes pido. The endings are the regular ones from the lessons on -ar verbs and -er and -ir verbs; only the stem vowel changes. The good news is that the change follows a small set of patterns and one rule about where it happens.

Quick shortcut: the stem vowel changes in yo, tú, usted/él/ella and ustedes/ellos/ellas, and stays the same in nosotros/as, vosotros/as and vos. There are three patterns: e → ie (querer → quiero), o → ue (poder → puedo, dormir → duermo) and e → i (pedir → pido).

Where the change happens

These verbs change their stem vowel only when the stress falls on it. In yo, tú, usted/él/ella and ustedes/ellos/ellas the stress is on the stem (PIEN-so, PIEN-san), so the vowel changes. In nosotros/as, vosotros/as and vos the stress moves to the ending (pen-SA-mos, pen-SÁIS, pen-SÁS), so the stem keeps its original vowel:

Subject pensar (e → ie) volver (o → ue) dormir (o → ue) pedir (e → i)
yo pienso vuelvo duermo pido
tú piensas vuelves duermes pides
vos pensás volvés dormís pedís
usted / él / ella piensa vuelve duerme pide
nosotros/as pensamos volvemos dormimos pedimos
vosotros/as pensáis volvéis dormís pedís
ustedes / ellos / ellas piensan vuelven duermen piden

A few things worth noticing:

  • Each verb keeps the endings of its own group: pensar takes the -ar endings, volver the -er endings, dormir and pedir the -ir endings. With -ir verbs, vos and vosotros/as share one form (dormís, pedís), just as with regular -ir verbs.
  • querer and poder follow exactly this pattern too. Their full forms are in the lesson on ir, hacer, querer and poder.
  • The change can combine with another irregularity: tener changes e → ie (tienes) and also has its own yo form (tengo), as explained in the tener lesson.

The three patterns

Pattern Verbs Example
e → ie querer (to want), pensar (to think), empezar (to begin), cerrar (to close), preferir (to prefer), entender (to understand), perder (to lose, to miss) Prefiero el té. (I prefer tea.)
o → ue poder (can, to be able to), dormir (to sleep), volver (to return, to come back), encontrar (to find), contar (to count, to tell), costar (to cost), recordar (to remember), almorzar (to have lunch) Vuelvo a casa a las seis. (I come back home at six.)
e → i pedir (to ask for, to order), repetir (to repeat), servir (to serve) Pido un café. (I ask for a coffee.)

The e → i pattern only happens in -ir verbs. The other two patterns occur in -ar, -er and -ir verbs.

Here are more examples in short sentences:

  • La tienda cierra a las ocho. (The shop closes at eight.) · La clase empieza a las nueve. (The class starts at nine.)
  • No entiendo. (I don't understand.) · Siempre pierdo las llaves. (I always lose my keys.)
  • Encuentro mis llaves. (I find my keys.) · No recuerdo su nombre. (I don't remember her name.)
  • ¿Cuánto cuesta? (How much does it cost?) · Almuerzo a las dos. (I have lunch at two.)
  • Ella repite la frase. (She repeats the sentence.) · El camarero sirve la comida. (The waiter serves the food.)

Using them with other verbs

querer, poder, preferir and pensar are often followed directly by an infinitive: Quiero dormir. (I want to sleep.) · Puedo ayudar. (I can help.) · Prefiero comer en casa. (I prefer to eat at home.) · Pienso viajar en verano. (I plan to travel in the summer.) pensar has other uses too: Pienso en mi familia. (I think about my family.)

empezar is different: it needs a before an infinitive, Empiezo a trabajar a las ocho. (I start working at eight.)

Not every verb changes

The infinitive alone doesn't tell you whether a verb changes its stem. Many verbs with an e or o in the stem are completely regular: comer → como, vender → vendo, aprender → aprendo. Dictionaries mark stem-changing verbs, for example pensar (ie) or dormir (ue). It helps to learn each new verb together with its yo form: pensar, pienso · dormir, duermo · pedir, pido.

Common mistakes

  • ❌ Yo dormo ocho horas. → ✅ Yo duermo ocho horas. (the stem vowel changes in yo)
  • ❌ Nosotros duermimos ocho horas. → ✅ Nosotros dormimos ocho horas. (nosotros keeps the plain o)
  • ❌ Ella pede un café. → ✅ Ella pide un café. (pedir changes e → i, not e → ie)
  • ❌ Ellos pensan en el viaje. → ✅ Ellos piensan en el viaje. (ustedes/ellos/ellas change the stem too)
  • ❌ Ella viende el coche. → ✅ Ella vende el coche. (vender is regular; not every verb with an e in its stem changes)
  • ❌ Empiezo trabajar a las ocho. → ✅ Empiezo a trabajar a las ocho. (empezar needs a before an infinitive)

Quick check

Which form of the verb in parentheses completes each sentence?

  1. Yo ___ ocho horas. (dormir)
  2. Nosotros ___ a casa a las seis. (volver)
  3. Ella ___ un café. (pedir)
  4. Yo no ___ ir hoy. (poder)
  5. Ellos ___ en su familia. (pensar)
Show answers
  1. Yo duermo ocho horas.   2. Nosotros volvemos a casa a las seis.   3. Ella pide un café.   4. Yo no puedo ir hoy.   5. Ellos piensan en su familia.

Key takeaways

  • Stem-changing verbs change the vowel in the stem when the stress falls on it: in yo, tú, usted/él/ella and ustedes/ellos/ellas, but not in nosotros/as, vosotros/as and vos.
  • The three patterns are e → ie (querer, pensar), o → ue (poder, dormir, volver) and e → i (pedir, repetir); e → i only occurs in -ir verbs.
  • The endings are the regular ones for each verb's group (-ar, -er or -ir).
  • The infinitive doesn't show whether a verb changes, so it helps to learn each verb with its yo form (dormir, duermo).
  • querer, poder, preferir and pensar are often followed directly by an infinitive; empezar takes a before one.