Remember that infinitives, the dictionary form of verbs, end in -ar, -er, or -ir in Spanish; the part of the verb that remains when you remove the infinitive ending is called the verb’s stem. There are three categories of stem-changing verbs, which all follow a similar pattern of changing the last vowel of their stem in the conjugated forms where that vowel is stressed. The verb endings are the same ones that we’ve learned already, according to whether it’s an -ar, -er or -ir verb.

Notice that the stem vowel in the infinitive changes in all three singular forms, as well as the third-person plural, but not in the first- or second-person plural. That is to say that when the verb’s subject is yo, tú, él, ella, usted, ellos, ellas,  or ustedes, then the stem vowel will change. But the stem vowel in the nosotros, nosotras, vosotros  and vosotras forms is the same as the infinitive.

We’ll start with the verbs that change from o to ue. Here’s an example of an o-ue verb, followed by a list of verbs that are conjugated with this pattern:

Dormir (to sleep)
	Singular	Plural
1a	duermo	dormimos
2a	duermes	dormís
3a	duerme	duermen

Verbos con cambio de raíz (stem-changing verbs) o – ue

• Almorzar (to eat lunch)
• Costar (to cost)
• Devolver (to return a thing, to give back)
• Encontrar (to find)
• Jugar (to play) (u-ue: juego, juegas, juega, jugamos, jugáis, juegan)
• Poder (to be able)
• Recordar (to remember)
• Volver (to return, to come back)

Some examples of stem-changing vowels used in complete sentences:

• ¿Puedes hablar español? (Can you speak Spanish?)
• Sí, puedo hablar un poco. (Yes, I can speak a little.)
• ¿A qué hora vuelven ustedes a casa? (What time do you -plural- return home?)
• Volvemos a casa a las cinco y media de la tarde. (We return home at 5:30 pm.)