The main form of the verb is called the infinitive. The infinitive consists of two parts: the ending and the stem. The ending is the last two letters. There are only three different endings: -ar, -er, and -ir. The stem is everything else, except the ending.

hablar: ending = ar, stem = habl
comer: ending = er, stem = com
vivir: ending = ir, stem =viv

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Some spanish verbs are called stem-changers because when they are conjugated, the stem changes in a predictable way. In one group of stem-changing verbs, the letter e in the stem changes to i in all but the nosotros and vosotros forms. This particular type of stem-changing verb is found only in -ir verbs.

pedir

yo pido
tú pides
él, ella, Ud. pide

nosotros/as pedimos
vosotros/as pedís

ellos, ellas, Uds. piden
For a more complete discussion of verbs that change e:i click here.

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Here are some common e:i stem-changing verbs.

competir
to compete

concebir
to conceive

despedir
to fire

impedir
to impede

medir
to measure

pedir
to ask for

reñir
to scold, quarrel

repetir
to repeat

servir
to serve

teñir
to color, dye, stain

vestir
to dress