Stem-changing verbs

All Spanish verbs have a stem (la raíz) and an ending (-ar, -er, -ir): pensar, volver, pedir. There is a large group of verbs that change their stem in the present tense when the stem vowel is stressed. There are three types: those that change -e to -ie-, those that change -o- to -ue- and those that change -e- to -i-:

to think:	to come back:	to ask (for):
pensar (ie) pienso piensas piensa pensamos pensáis piensan	volver (ue) vuelvo vuelves vuelve volvemos volvéis vuelven	pedir (i) pido pides pide pedimos pedís piden

Common verbs following these patterns in the present tense:
e > ie: cerrar, comenzar, empezar, entender, pensar, perder, preferir, querer, sentir
e > i: elegir, (im)pedir, seguir, servir, (son)reír
o > ue: contar, dormir, encontrar, morir, mostrar, poder, resolver, volar, volver

Jugar has a different stem change, from u to ue in the same places:
juego, juegas, juega, jugamos, jugáis, juegan.

Note: All -ir stem-changing verbs change their stems in other tenses (see appendix A).

For exercises, click here. Do also 'Ponte a punto' exercises 1 and 2 in page 88 in the grammar book.