Stem-changing verbs: why poder becomes puedo

Puedo, pienso, pido: the vowel that moves, and when

What are the four stem-change patterns?

Pattern	Example	More verbs like it
e → ie	pensar: pienso	querer, cerrar, entender, perder, empezar
o → ue	poder: puedo	dormir, recordar, encontrar, volver, mover
e → i	pedir: pido	repetir, seguir, servir, elegir
u → ue	jugar: juego	none: jugar is the only one

Here is each pattern in a sentence from nemiria’s Present Simple card:

• Él piensa en su futuro constantemente.He thinks about his future constantly.
• No puedo dormir cuando hay ruido.I cannot sleep when there is noise.
• Pido ayuda cuando la necesito.I ask for help when I need it.
• Él juega al fútbol todos los sábados.He plays football every Saturday.

Why does nosotros stay the same?

That is why the change skips the same forms in every stem-changing verb. Here are poder, to be able to, pensar, to think, and pedir, to ask for, in the Present Simple:

Person	poder	pensar	pedir
yo	puedo	pienso	pido
tú	puedes	piensas	pides
él / ella	puede	piensa	pide
nosotros	podemos	pensamos	pedimos
vosotros	podéis	pensáis	pedís
ellos / ellas	pueden	piensan	piden

Usted takes the él / ella form and ustedes the ellos / ellas form; vosotros is used in Spain.

Many teachers call this the “boot” pattern: in a six-row table, the changed forms draw the outline of a boot around nosotros and vosotros.

Which tenses does the stem change affect?

The Present Simple, as above, in every stem-changing verb.

The Present Subjunctive, and the commands built from it. The same four forms change: Espero que puedas venir a la boda, I hope you can come to the wedding. In -ir verbs, nosotros and vosotros change too, with the narrower vowel described below: pidamos, durmamos. nemiria’s note on empezar gives the command forms empiece and empiecen.

Not the Preterite, for -ar and -er verbs. Their past is regular: Pensaste en ello toda la noche, you thought about it all night. (Poder and querer do have irregular preterites, pude and quise, but that is a separate irregularity, not the stem change.)

What is the extra change in -ir verbs?

Stem-changing verbs ending in -ir change once more, in the Preterite for él and ellos, in the -ndo form, and in the nosotros and vosotros forms of the Present Subjunctive. The vowel narrows: e becomes i, and o becomes u.

So dormir has three stems to know: duermo in the present, durmió in the Preterite, and dormimos in most of the other forms.

How do you know which pattern a verb follows?

The infinitive does not tell you. Pensar changes and hablar does not, and nothing in the spelling warns you. Two things help:

e → i only happens in -ir verbs. An -ar or -er verb with a stem change is e → ie or o → ue (or u → ue, for jugar). Some -ir verbs are e → ie too, like sentir (siento), and then they use e → i for the extra change: sintió.

Learn them as a group. Stem-changers are some of the most common verbs in Spanish: poder, querer, pensar, dormir, volver, jugar. Meeting them together makes the pattern stick faster than meeting them one at a time.

A few also carry a small spelling change to keep the sound right, such as jugué and empecé in the Preterite yo form. The reflexive verbs follow the same patterns: me despierto, me acuesto, me visto.

How should you practise stem-changing verbs?

Say the forms aloud. The rule is about stress, and you hear stress more easily than you see it. Puedo, puedes, puede, podemos, pueden: the jump is obvious when you say it.

Drill one pattern at a time. Settle o → ue before you add e → ie, and leave e → i until both feel easy.

Watch nosotros. The most common mistake is changing it anyway: puedemos instead of podemos.