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 third group of stem-changing verbs, the letter e in the stem changes to i in all forms except the nosotros and vosotros.

repetir

repito
repites
repite

repetimos
repetís

repiten

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Here's another e:i stem changing verb. Compare it to the regular verb vivir. Notice that the endings are the same for regular verbs and stem-changing verbs.

competir (e:i)
vivir (regular -ir verb)

compito
vivo

compites
vives

compite
vive

competimos
vivimos

competís
vivís

compiten
viven

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

pedir (e:i)

pido
pides
pide

pedimos
pedís

piden

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Here is a list of common e:i stem-changing verbs.

bendecir
to bless impedir
to impede
colegir
to deduce maldecir
to curse
competir
to compete medir
to measure
conseguir
to get, obtain pedir
to ask for, to order
corregir
to correct perseguir
to pursue, to persecute
decir
to say, tell reír
to laugh
despedir
to dismiss, fire repetir
to repeat
elegir
to elect seguir
to follow, continue
freír
to fry servir
to serve
gemir
to groan, moan sonreír
to smile

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Let's add a flashcard for e:i stem-changers:

Verb Flashcards

e:i stem-changers (repetir)

repito
repites
repite
repetimos
repetís
repiten

(decir, despedir, medir, pedir, servir)