An irregular verb is one that does not take the -ed ending for the Past Simple and Past Participle forms. Some irregular verbs do not change; put put put, while others change completely; buy bought bought, etc. Unfortunately for the student, there are a lot or irregular verbs and they include many of the commonest verbs in the English language.
Irregular verbs fall into 5 categories:
Auxiliary Verb; Ditransitive Verb; Dynamic Verb; Finite Verb; Inchoative Verb; Intransitive Verb; Modal Verb; Non-finite Verb; Phrasal Verb; Regular Verb; Stative Verb; Transitive Verb; Verb Group; Verb Phrase