When it comes to conjugating Spanish verbs, most will simply follow the basic rules: drop the infinitive ending and add the one appropriate to the verb's subject, tense, and mood. However, there are some verbs that, for one reason or another, do not conform to this simple behavior. We call these verbs irregular verbs. Some verbs are only irregular in a few conjugations, while others reliably refuse to conform in any situation.
Quite a number of verbs are irregular in the present indicative tense, and they deserve a chapter of their own. Verb irregularities in other tenses will be covered in the chapters that introduce those particular tenses.