A regular verb is a verb whose conjugation can be predicted given a few verb forms (principal parts) and a few rules. (Of course, the appreciation of how many are really only "a few" is subjective.) A verb is a part of speech that usually denotes action (bring, read), occurrence (to decompose (itself), to glitter), or a state of being (exist, live, soak, stand). Depending on the language, a verb may vary in form according to many factors, possibly including its tense, aspect, mood and voice. ... In linguistics, conjugation is the creation of derived forms of a verb from its principal parts by inflection (regular alteration according to rules of grammar). ... In language learning, the principal parts of a verb are the series of key forms which the student has to learn by heart in order to be able to conjugate the verb through all its forms. ...

A verb that cannot be conjugated like this is called an irregular verb. All natural languages, to different extents, have a number of irregular verbs. Auxiliary languages usually have a single regular pattern for all verbs (as well as other parts of speech) as a matter of design. Other constructed languages need not show such regularity, especially if they are designed to look similar to natural ones. 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 term natural language is used to distinguish languages spoken by humans for general-purpose communication from constructs such as computer-programming languages or the languages used in the study of formal logic, especially mathematical logic. ... An international auxiliary language (sometimes abbreviated as IAL or auxlang) is a language used (or to be used in the future) for communication between people from different nations who do not share a common native language. ... In grammar, a part of speech or word class is defined as the role that a word (or sometimes a phrase) plays in a sentence. ... An artificial or constructed language (known colloquially as a conlang among aficionados), is a language whose phonology, grammar and vocabulary are specifically devised by an individual or small group, rather than having naturally evolved as part of a culture as with natural languages. ...

The most simple form of regularity involves a single class of verbs, a single principal part (the root or a conjugated form in a given person, number, tense, aspect, mood, etc.), and a set of unique rules to produce each form in the verb paradigm. More complex regular patterns may have several verb classes (e. g. distinguished by their infinitive ending), more than one principal part (e. g. the infinitive and the first person singular, present tense, indicative mood), and more than one type of rule (e. g. rules that add suffixes and other rules that change the vowel in the root). The root is the primary lexical unit of a word, which carries the most significant aspects of semantic content and cannot be reduced into smaller constituents. ...

Sometimes it is highly subjective to state whether a verb is regular or not. For example, if a language has ten different conjugation patterns and two of them only comprise five or six verbs each while the rest are much more populated, it is a matter of choice to call the verbs in the smaller groups "irregular".