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
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 o in the stem changes to ue in all but the nosotros and vosotros forms. This particular type of stem-changing verb is found in all three verb types.
contar
yo cuento
tú cuentas
él, ella, Ud. cuenta
nosotros/as contamos
vosotros/as contáis
ellos, ellas, Uds. cuentan
For a more complete discussion of verbs that change o:ue click here.
Here are some common o:ue stem-changing verbs.
acordar
to agree, decide
acostar
to put to bed
almorzar
to eat lunch
aprobar
to approve
colgar
to hang
contar
to count, to tell
costar
to cost
demostrar
to demonstrate, prove
devolver
to return an object
dormir
to sleep
encontrar
to find
envolver
to wrap
forzar
to force
morder
to bite
morir
to die
mostrar
to show
mover
to move an object, to motivate
poder
to be able to
probar
to test, taste
recordar
to remember
resolver
to solve
rogar
to beg, pray
soler
to be in the habit of
sonar
to sound, ring
so?ar
to dream
tostar
to toast
volar
to fly
volver
to return
