Remember, there are three types of infinitives: -ar, -er, -ir. Infinitives are made up of two parts: the ending and the stem. In the following examples, the stem is underlined and the ending is in bold.

hablar
comer
vivir

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With regular verbs, the stem stays the same, and the ending changes as they are conjugated.

hablo como vivo
hablas comes vives
habla come vive

hablamos comemos vivimos
habláis coméis vivís

hablan comen viven

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With some verbs, the stem also changes when you conjugate them. In the present tense, there are three groups of stem-changing verbs:

o:ue
e:ie
e:i

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With the first group of stem-changing verbs, the letter o in the stem changes to ue in all forms except the nosotros and vosotros.

contar

cuento
cuentas
cuenta

contamos
contáis

cuentan

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Here's another o:ue stem changing verb. Compare it to the regular verb comer. Notice that the endings are the same for regular verbs and stem-changing verbs.

mover (o:ue)
comer (regular -er verb)

muevo
como

mueves
comes

mueve
come

movemos
comemos

movéis
coméis

mueven
comen

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Here's another o:ue stem-changing verb. Notice how the stem doesn't change in the nosotros and vosotros forms.

dormir (o:ue)

duermo
duermes
duerme

dormimos
dormís

duermen

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Here is a list of common o:ue stem-changing verbs.

almorzar
to eat lunch morir
to die
aprobar
to approve mostrar
to show
colgar
to hang mover
to move (an object)
contar
to count, to tell
probar
to prove, test, sample, taste
costar
to cost recordar
to remember
devolver
to return (an object) resolver
to solve

volver
to return (from someplace) rogar
to beg, pray
dormir
to sleep sonar
to sound, ring
encontrar
to find soñar (con)
to dream (about)
envolver
to wrap tostar
to toast
morder
to bite volar
to fly

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Let's add a flashcard for o:ue stem-changers:

Verb Flashcards

o:ue stem-changers (dormir)

duermo
duermes
duerme
dormimos
dormís
duermen

(almorzar, contar, costar, volver, recordar, volar)