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

cerrar

cierro
cierras
cierra

cerramos
cerráis

cierran

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

entender (e:ie)
comer (regular -er verb)

entiendo
como

entiendes
comes

entiende
come

entendemos
comemos

entendéis
coméis

entienden
comen

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

mentir (e:ie)

miento
mientes
miente

mentimos
mentís

mienten

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

acertar
to guess, get right encender
to light, kindle
advertir
to advise, warn entender
to understand
cerrar
to close, shut fregar
to scrub, wash dishes
comenzar
to begin hervir
to boil
confesar
to confess mentir
to lie
consentir
to consent negar
to deny
convertir
to convert pensar (en)
to think about
defender
to defend perder
to lose
empezar
to begin preferir
to prefer

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

Verb Flashcards

e:ie stem-changers (cerrar)

cierro
cierras
cierra
cerramos
cerráis
cierran

(comenzar, empezar, entender, pensar, perder, preferir)