TRADITIONALLY IRREGULAR
SPANISH VERBS

According to A New Reference Grammar of Modern Spanish* there are only two dozen Spanish verbs (not counting compound verbs formed from them) that are traditionally defined as truly "irregular".  Not all of these verbs are irregular in all forms.  No verb can be truly considered to be irregular in the Imperfect Subjunctive mood as you conjugate all verbs the same way regardless of whether they are considered to be "regular" or "irregular".  However, because you form the Imperfect Subjunctive mood from the third person plural (ellos) of the Preterite, which may be "irregular" such Imperfect Subjunctive formations could also be considered "irregular".

Don't confuse these verbs with those that are "radical changing" verbs.  Radical changing verbs have regular endings, but a vowel in the stem is modified in some forms.  For example, "contar - to count", is conjugated in the present indicative form as: cuento, cuentas, cuenta, contamos, contáis, cuenten. There is unfortunately no clue exists to tell you whether or not a verb is a radically changing verb.  And it is not necessarily a radically changing verb in all tenses and moods.  Contar for example is only a radically changing verb in the present indicative tense and the present subjunctive mood.  The verb "dormir - to sleep" is a radically changing verb in those forms and several others.

These verbs are considered to be traditionally irregular (but not in all forms):

Notice that most of these verbs are quite commonly used, so you will have to pay close attention to learning their conjugations.

*A New Reference Grammar of Modern Spanish, John Butt & Carmen Benjamin, pages 163-164