1. Stem-changing verbs
Apuntes de gramática

2. What is a stem-changing verb?
A stem-changing verb is considered irregular, however the only part of the verb that is irregular is the stem.
A stem-changing verb still uses regular –AR, -ER, or –IR verb endings.

3. Ch. 7 Stem-changing verbs
We’ve already covered 2 stem-changing verbs in this chapter, JUGAR and DORMIR.
The rule to the irregular stem for JUGAR is that the u changes to ue (in all forms except nosotros).
jueg-
The rule to the irregular stem for DORMIR is similar: the o changes to ue (in all forms except nosotros).
duerm-

4. Ch. 7 Stem-changing verbs
In addition to JUGAR and DORMIR, verbs that we will use in this unit that are stem-changing verbs are:
EMPEZAR (To begin)
QUERER (To want)
PERDER (To lose)
PREFERIR (To prefer)
The e of the stem changes to ie in all forms except nosotros (which stays regular).

5. Ch. 7 Stem-changing verbs
These two verbs follow the same stem-change pattern as DORMIR:
PODER (To be able to)
VOLVER (To return)
The “o” changes to a “ue” in all forms except nosotros.
pued-
vuelv-

6. IMPORTANTE
The verbs EMPEZAR, QUERER, and PREFERIR are often followed by an infinitive.
Ellos quieren ir al gimnasio.
They want to go to the gym.
¿Por qué prefieres jugar al fútbol?
Why do you prefer to play soccer?