A Spanish irregular verb is a verb that suffers a change in its stem when it is conjugated.
A verb stem is the root of a verb. For example, in the verb amar, am is the stem, and ar is the ending.
In Spanish, all verbs end in one of three ways: -ar, -er, -ir. Eliminate the ending of a verb, and all you are left with is its stem.
In Spanish, a verb is considered irregular when there is a change in its stem when conjugated. If the stem of a verb does not change when the verb is conjugated, then, it is considered a regular verb.
Let’s practice a bit! What’s the stem of the verb querer (to love)? It’s quer. When conjugated, the verb querer changes to quiero (I love), for example. Is querer a regular or an irregular verb in Spanish? Yes, it’s irregular! Its stem changes, for the vowel i is added in quer.
Oh! The verbs we are about to study are also called “boot verbs.” Have you ever wondered why? It’s because when they are conjugated and placed in a conjugation table, the only verbs that suffer a stem change form a boot.
That kind of looks like a boot, doesn’t it?
Notice in the chart that the nosotros and vosotros forms are not affected by this stem change in the present tense. The vos form is not affected by this change either.