Stem-changing verbs modify their internal vowel in the stressed syllable of the present tense, but only in the singular forms and the third-person plural. The nosotros and vosotros forms are unchanged, which is a hallmark of this group. There are three main patterns: o to ue (as in poder and dormir), e to ie (as in pensar and querer), and e to i (as in pedir). The verb dormir (to sleep) shifts to duermo, duermes, duerme, dormimos, dormís, duermen.
Other verbs that follow the o to ue pattern include encontrar, recordar, contar, costar, mover, volver, and morir, joining the already-irregular poder in this productive family. The e to ie group is exemplified by pensar (to think): pienso, piensas, piensa, pensamos, pensáis, piensan. This same pattern applies to querer, empezar, cerrar, entender, perder, sentir, and mentir. The third type, e to i, is illustrated by pedir (to ask for): pido, pides, pide, pedimos, pedís, piden. This e to i shift is rarer but equally important to recognize in context.
A special case is jugar (to play), which uniquely changes u to ue rather than the more common u to ue pattern of other verbs: juego, juegas, juega, jugamos, jugáis, juegan. Mastering these stem changes unlocks dozens of related verbs, since the conjugation pattern stays consistent across the family. When a stem-changing verb is also irregular in the yo form (such as tener, poder, or decir), both modifications apply together.