Irregular verbs are verbs which do not follow normal rules for conjugation, they are common verbs in the English language that do not follow the simple system of adding “d” or “ed” to the end of the word to form the past tense.

According to "Englishpage.com" they are 680 irregular verbs so far including prefixed verbs (misunderstand, reread) as well as rare and antiquated forms (colorbreed, bethink). The most common irregular verbs in English are: be, have, say, do, know, get, see, think, go and take.