Most of the English verbs are conjugated in various tenses and persons, according to regular patterns:
All the verbs that do not follow the rules above in the conjugation of one or more tenses are called irregular verbs. There are about 200 English irregular verbs, many of them very common.
Most of the irregular verbs conjugate without following the rules in simple past and past participle.
Although there are many irregular verbs in English, learning a few for each type of variation will help a non-native speaker know how to conjugate most of them.