Present indicative — irregular stem changes in Spanish (A2)

what is it?

Many Spanish verbs take the normal present-tense endings but change their stem in certain persons. The most common pattern is the boot: the stem vowel changes in every person except nosotros and vosotros. Other verbs have an irregular yo form only, a spelling shift to keep the sound, or a written accent on a stressed vowel.

when to use it

e→ie: pensar → pienso, querer → quiero, tener → tengo/tienes.
o→ue: poder → puedo, dormir → duermo. Also jugar → juego (u→ue) and oler → huelo.
e→i: pedir → pido, seguir → sigo (only in -ir verbs).
irregular yo: saber → sé, dar → doy, caber → quepo.
spelling in yo: verbs in -ger, -gir, -guir change before -o: coger → cojo, elegir → elijo, seguir → sigo, vencer → venzo.
written accent: enviar → envío, actuar → actúo, reír → río, reunir → reúno, prohibir → prohíbo — but cambiar → cambio (no accent).
y-insertion: verbs in -uir add y inside the boot: construir → construyo.

formation

Keep the regular endings; change the stem. In a boot verb the change happens everywhere except nosotros and vosotros, which keep the plain stem.

person	pensar (e→ie)	poder (o→ue)	pedir (e→i)
yo	pienso	puedo	pido
tú	piensas	puedes	pides
él/ella	piensa	puede	pide
nosotros	pensamos	podemos	pedimos
vosotros	pensáis	podéis	pedís
ellos/ellas	piensan	pueden	piden

Verbs with an irregular yo form, a spelling shift, an accent, or y show the change in the first person; the other persons follow the stem's own pattern or stay regular.

infinitive	yo form	meaning
saber	sé	I know
dar	doy	I give
coger	cojo	I take/catch
elegir	elijo	I choose
seguir	sigo	I follow/continue
enviar	envío	I send
actuar	actúo	I act
reunir	reúno	I gather
construir	construyo	I build
oler	huelo	I smell

The accent and the y also live inside the boot: envías, envía, envían and construyes, construye, construyen, but enviamos / construimos stay plain.

examples in context

Pienso que tienes razón.

I think you're right.

e→ie in pienso.

Ella puede venir mañana, pero yo no puedo.

She can come tomorrow, but I can't.

o→ue in both forms of poder.

Pido un café con leche.

I'll have / I ask for a coffee with milk.

e→i in pido.

Yo sé cocinar, pero no sé bailar.

I know how to cook, but I don't know how to dance.

Irregular yo: sé, never sabo.

Te doy mi número de teléfono.

I'll give you my phone number.

Irregular yo: doy.

Te envío las fotos y ellos construyen la casa.

I'll send you the photos and they build the house.

Accent in envío, y-insertion in construyen.

La cocina huele a pan y los niños se ríen.

The kitchen smells of bread and the children laugh.

Note the h of huele and the accent in ríen.

Los viernes reúno a todo el equipo.

On Fridays I gather the whole team.

Accent on the ú: reúno.

common pitfalls

Applying the boot change to nosotros/vosotros. Write pensamos, not piensamos — those two persons keep the plain stem.
Regularizing an irregular yo-form. It is sé and doy, never sabo or do.
Forgetting the spelling shift in the yo-form of -ger/-gir/-guir verbs. Write cojo and sigo, not cogo or seguo.
Dropping the accent or the y in -iar/-uar/-uir verbs. It is envío, actúo, construyo — not envio, actuo, construo.
Mishandling oler. The diphthong needs its h: huelo, not uelo or olo.
Leaving the hiatus without its accent. Write sonrío, reúno, prohíbo, not sonrio, reuno, prohibo.

Quick check

Three sentences, graded here. Nothing is saved and no account is needed.

1 / 3

El conductor siempre ___ la misma ruta por la autopista. (seguir)

Fill in the blank with the correct present-tense form of the verb in parentheses.