A regular verb is any verb whose conjugation follows the typical grammatical inflections of the language to which it belongs. 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. 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). 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". The concept of regular and irregular verbs belongs mainly in the context of second language acquisition, where the defining of rules and listing of exceptions is an important part of foreign language learning. The concepts can also be useful in psycholinguistics, where the ways in which the human mind processes irregularities may be of interest. However, most other branches of linguistics do not use these categories; historical/comparative linguistics is more interested in categories such as strong and weak.
Although the causes of irregular verbs are almost exclusively historical, the way we process them is a matter for synchronic analysis and especially psycholinguistics. A common error for small children is to conjugate irregular verbs as though they were regular. This is regarded as evidence that we learn and process our native language partly by the application of rules, rather than, as some earlier scholarship had postulated, solely by learning the forms. In fact, children often use the most common irregular verbs correctly in their earliest utterances but then switch to incorrect regular forms for a time when they begin to operate systematically. That allows a fairly precise analysis of the phases of this aspect of first language acquisition.