In contrast to regular verbs, irregular verbs are those verbs that fall outside the standard patterns of conjugation in the languages in which they occur. The idea of an irregular verb is important in second-language acquisition, where the verb paradigms of a foreign language are learned systematically, and exceptions listed and carefully noted. Thus for example a school French textbook may have a section at the back listing the French irregular verbs in tables. Irregular verbs are often the most commonly used verbs in the language. In linguistic analysis, the concept of an irregular verb is most likely to be used in psycholinguistics, and in first-language acquisition studies, where the aim is to establish how the human brain processes its native language. One debate among 20th-century linguists revolved around the question of whether small children learn all verb forms as separate pieces of vocabulary or whether they deduce forms by the application of rules.[1] Since a child can hear a regular verb for the first time and immediately reuse it correctly in a different tense which he or she has never heard, it is clear that the brain does work with rules, but irregular verbs must be processed differently. Historical linguists rarely use the category irregular verb. Since most irregularities can be explained historically, these verbs are only irregular when viewed synchronically, not when seen in their historical context. When languages are being compared informally, one of the few quantitative statistics which are sometimes cited is the number of irregular verbs. These counts are not particularly accurate for a wide variety of reasons, and academic linguists are reluctant to cite them. But it does seem that some languages have a greater tolerance for paradigm irregularity than others.

n linguistics, conjugation is the creation of derived forms of a verb from its principal parts by inflection (alteration of form according to rules of grammar). Conjugation may be affected by person, number, gender, tense, aspect, mood, voice, or other grammatical categories. Typically the principal parts are the root and/or several modifications of it (stems). All the different forms of the same verb constitute a lexeme, and the canonical form of the verb that is conventionally used to represent that lexeme (as seen in dictionary entries) is called a lemma. The term conjugation is applied only to the inflection of verbs, and not of other parts of speech (inflection of nouns and adjectives is known as declension). Also it is often restricted to denoting the formation of finite forms of a verb – these may be referred to as conjugated forms, as opposed to non-finite forms, such as the infinitive or gerund, which tend not to be marked for most of the grammatical categories. Conjugation is also the traditional name for a group of verbs that share a similar conjugation pattern in a particular language (a verb class). For example, Latin is said to have four conjugations of verbs. This means that any regular Latin verb can be conjugated in any person, number, tense, mood, and voice by knowing which of the four conjugation groups it belongs to, and its principal parts. A verb that does not follow any of the standard conjugation patterns of the language is said to be an irregular verb. The system of all conjugated variants of a particular verb or class of verbs is called a verb paradigm; this may be presented in the form of a conjugation table.