1. The Present Tense of Stem-Changing Verbs:
e  ie, e  i, o  ue

2. There are three types or classes of “boot” verbs.
There is a fairly large group of verbs in Spanish that undergo changes in their stem when conjugated in the present tense.
These changes occur in all forms but the nosotros and the vosotros forms.
In a normal “verb chart”, when a line is drawn around the forms that change, the resulting shape vaguely resembles a boot or high-top shoe; thus, these verbs are sometimes informally called “boot” or “shoe” verbs.
There are three types or classes of “boot” verbs.

3. We’ll use preferir, to understand, as an example of this type.
Type 1: e  ie
We’ll use preferir, to understand, as an example of this type.
preferir
prefiero
prefieres
prefiere
preferimos
preferís
prefieren
*Note: There are 2 e’s in the stem – the one that changes is the 2nd one, the one closest to the ending.

4. Here are some other common e  ie verbs:
empezar, comenzar
to begin
pensar (+ infinitive)
to think; to plan (to do something)
perder
to lose
preferir
to prefer
querer
to want, to love

5. Type 2: e  i servir sirvo sirves sirve servimos servís sirven
We’ll use servir, to serve, as an example of this type.
servir
sirvo
sirves
sirve
servimos
servís
sirven
*Note: All e  i verbs are -ir verbs.

6. Here are some other common e  i verbs:
pedir
to ask for, to request
repetir
to repeat, to have a second helping

7. We’ll use almorzar, to have lunch, as an example of this type.
Type 3: o  ue
We’ll use almorzar, to have lunch, as an example of this type.
almorzar
almuerzo
almuerzas
almuerza
almorzamos
almorzáis
almuerzan

8. *Costar is normally used only in the third persons.
Here are some other common o  ue verbs:
costar*
to cost
dormir
to sleep
encontrar
to find
poder
to be able; can
recordar
to remember
soñar (con)
to dream (about)
volver
to return, to come back
*Costar is normally used only in the third persons.