Some of the verbs we have introduced previously as irregular are actually stem-changing verbs. This means that there is a small spelling change in the stem of the verb. However, these verbs do follow a pattern:
 

e > ie
PENSAR (to think)
pienso	pensamos
piensas	pensáis
piensa	piensan

 

o or u > ue
VOLVER (to return)
vuelvo	volvemos
vuelves	volvéis
vuelve	vuelven

 

e > i (ir verbs only)
PEDIR (to ask for)
pido	pedimos
pides	pedís
pide	piden

Notice that the nosotros and vosotros forms of the verbs do not have stem changes, whereas the others do.

More Stem-Changing Verbs
 

e > ie

cerrar (to close)

empezar(to begin)

pensar(to think)

perder( to lose)

preferir(to prefer (Ch. 6))

querer(to want (Ch 6))

 

o or u > ue

almorzar(to eat lunch)

dormir(to sleep)

jugar(to play (games))

poder(to be able)

volver( to return (Ch. 6))

 

e > i

pedir(to ask for)

servir(to serve)