In Spanish, a verb is considered irregular if it does not follow the regular conjugation pattern for verbs ending in -ar, -er, or -ir. Irregular verbs in Spanish have changes in the stem or the ending of the verb in some or all of the conjugations.
There are several different types of irregular verbs in Spanish, but some of the most common irregular verbs include:
1. Stem-changing verbs: These verbs have a vowel change in the stem in some or all of the conjugations. For example, the verb "pensar" (to think) changes from "pienso" in the first person singular to "piensas" in the second person singular.
2. Irregular yo-form verbs: These verbs have an irregular conjugation only in the first person singular (yo) form. For example, the verb "ser" (to be) changes from "soy" in the first person singular to "eres" in the second person singular.
3. Completely irregular verbs: These verbs have an irregular conjugation in all of the forms. For example, the verb "ir" (to go) changes from "voy" in the first person singular to "van" in the third person plural.
It's important to note that irregular verbs in Spanish are very common, so it's important to study them and learn their conjugation patterns.