Spanish Stem-Changing Verbs (e→ie, o→ue, e→i)

Questions about this deck

How do I know if a Spanish verb is stem-changing?

You can't tell from the infinitive: pensar changes (pienso) but pesar doesn't (peso), and volver changes (vuelvo) while comer doesn't. Learn each verb together with its pattern. Dictionaries mark it in brackets after the entry — pedir (i), contar (ue), querer (ie) — and that is exactly what the notes in this deck give you.

Why don't nosotros and vosotros change the stem?

Because the change only happens when the stress falls on the stem vowel. In quiero, quieres, quiere and quieren the stress is on the stem, so e becomes ie. In queremos and queréis the stress moves to the ending, so the stem keeps the infinitive vowel. Writing the six forms in a boot shape around the four changing forms is a quick way to check yourself.

Do these verbs also change in the past tense?

Only the -ir ones. Verbs like dormir, morir, pedir, servir, sentir, preferir and divertirse change again in the third person of the preterite and in the gerund: durmió, durmieron, durmiendo; pidió, pidieron, pidiendo; prefirió, prefirieron, prefiriendo. Stem-changing -ar and -er verbs such as pensar, cerrar and volver keep the infinitive stem in the preterite (pensé, cerré, volví); poder, tener, venir and decir are a separate story, with their own irregular preterites (pude, tuve, vine, dije).

Is jugar really the only u→ue verb?

Yes, jugar is the only verb with u→ue: juego, juegas, juega, jugamos, jugáis, juegan. Almost everything else follows e→ie, o→ue or e→i; the only other rarity is i→ie in adquirir (adquiero) and inquirir, which you won't need at A2. Note the preposition too: in Spain people say juego al tenis, while in Mexico and much of Latin America juego tenis is normal.