Regular and irregular verbs
A regular verb is any verb whose conjugation follows the typical pattern, or one of the typical patterns, of the language to which it belongs. A verb whose conjugation follows a different pattern is called an irregular verb. This is one instance of the distinction between regular and irregular inflection, which can also apply to other word classes, such as nouns and adjectives. In English, for example, verbs such as "play", "enter", and "like" are regular since they form their inflected parts by adding the typical endings "-s", "-ing" and "-ed" to give forms such as  plays, "entering", and "liked". On the other hand, verbs such as "drink", "hit" and "have" are irregular since some of their parts are not made according to the typical pattern: "drank" and "drunk" (not "drinked"); "hit" (as past tense and past participle, not "hitted") and "has" and "had" (not "haves" and "haved"). The classification of verbs as regular or irregular is to some extent a subjective matter. If some conjugational paradigm in a language is followed by a limited number of verbs, or it requires the specification of more than one principal part (as with the German strong verbs), views  may differ as to whether the verbs in question should be considered irregular. Most inflectional irregularities arise as a result of series of fairly uniform historical changes so forms that appear to be irregular from a synchronic (contemporary) point of view may be seen as following more regular patterns when the verbs are analyzed from a diachronic (historical linguistic) viewpoint. Development. When a language develops some type of inflection, such as verb conjugation, it normally produces certain typical (regular) patterns by which words in the given class come to make their inflected forms. The language may develop a number of  different regular patterns, either as a result of conditional sound changes which cause differentiation within a single pattern, or through patterns with different derivations coming to be used for the same purpose. An example of the latter is provided by the strong and weak verbs of the Germanic languages; the strong verbs inherited their method of making past forms (vowel ablaut) from Proto-Indo-European, while for the weak verbs a different method (addition of dental suffixes) developed. Irregularities in verb conjugation (and other inflectional irregularities) may arise in various ways. Sometimes the result of multiple conditional and selective historical  sound changes is to leave certain words following a practically unpredictable pattern. This has happened with the strong verbs (and some groups of weak verbs) in English; patterns such as "sing–sang–sung" and "stand–stood–stood", although they derive from what were more or less regular patterns in older languages, are now peculiar to a single verb or small group of verbs in each case, and are viewed as irregular. Irregularities may also arise from suppletion – forms of one verb may be taken over and used as forms of another. This has happened in the case of the English  word "went", which was originally the past tense of "wend", but has come to be used instead as the past tense of "go". The verb "be" also has a number of suppletive forms ("be", "is", "was", etc., with various different origins) – this is common for copular verbs in Indo-European languages. The regularity and irregularity of verbs is affected by changes taking place by way of analogy – there is often a tendency for verbs to switch to a different, usually more regular, pattern under the influence of other verbs. This is less likely when the existing forms are very familiar  through common use – hence among the most common verbs in a language (like "be", "have", "go", etc.) there is often a greater incidence of irregularity. (Analogy can occasionally work the other way, too – some irregular English verb forms such as "shown", "caught" and "spat" have arisen through the influence of existing strong or irregular verbs.) Types of pattern. The most straightforward type of regular verb conjugation pattern involves a single class of verbs, a single principal part (the root or one particular conjugated form), and a set of exact rules which produce, from that principal part, each of the remaining  forms in the verb's paradigm. This is generally considered to be the situation with regular English verbs – from the one principal part, namely the plain form of a regular verb (the bare infinitive, such as "play", "happen", "skim", "interchange", etc.), all the other inflected forms (which in English are not numerous; they consist of the third person singular present tense, the past tense and past participle, and the present participle/gerund form) can be derived by way of consistent rules. These rules involve the addition of inflectional endings ("-s", "-[e]d", "-ing"), together with certain morphophonological rules about how those  endings are pronounced, and certain rules of spelling (such as the doubling of certain consonants). Verbs which in any way deviate from these rules (there are around 200 such verbs in the language) are classed as irregular. A language may have more than one regular conjugation pattern. French verbs, for example, follow different patterns depending on whether their infinitive ends in "-er", "-ir" or "-re" (complicated slightly by certain rules of spelling). A verb which does not follow the expected pattern based on the form of its infinitive is considered irregular. In some languages, however, verbs may be considered regular  even if the specification of one of their forms is not sufficient to predict all of the rest; they have more than one principal part. In Latin, for example, verbs are considered to have four principal parts (see Latin conjugation for details). Specification of all of these four forms for a given verb is sufficient to predict all of the other forms of that verb – except in a few cases, when the verb is irregular. To some extent it may be a matter of convention or subjective preference to state whether a verb is regular or irregular. In English, for  example, if a verb is allowed to have three principal parts specified (the bare infinitive, past tense and past participle), then the number of irregular verbs will be drastically reduced (this is not the conventional approach, however). The situation is similar with the strong verbs in German (these may or may not be described as irregular). In French, what are traditionally called the "regular "-re" verbs" (those that conjugate like "vendre") are not in fact particularly numerous, and may alternatively be considered to be just another group of similarly behaving irregular verbs. The most unambiguously irregular verbs are often very  commonly used verbs such as the copular verb "be" in English and its equivalents in other languages, which frequently have a variety of suppletive forms and thus follow an exceptionally unpredictable pattern of conjugation. Irregularity in spelling only. It is possible for a verb to be regular in pronunciation, but irregular in spelling. Examples of this are the English verbs "lay" and "pay". In terms of pronunciation, these make their past forms in the regular way, by adding the sound. However their spelling deviates from the regular pattern: they are not spelt (spelled) "layed" and "payed" (although the latter form  is used in some e.g. nautical contexts as "the sailor payed out the anchor chain"), but "laid" and "paid". This contrasts with fully regular verbs such as "sway" and "stay", which have the regularly spelt past forms "swayed" and "stayed". The English present participle is never irregular in pronunciation, with the exception that "singeing" irregularly retains the "e" to distinguish it from "singing". Linguistic study. In linguistic analysis, the concept of regular and irregular verbs (and other types of regular and irregular inflection) commonly arises in psycholinguistics, and in particular in work related to language acquisition. In studies of first  language acquisition (where the aim is to establish how the human brain processes its native language), one debate among 20th-century linguists revolved around whether small children learn all verb forms as separate pieces of vocabulary or whether they deduce forms by the application of rules. Since a child can hear a regular verb for the first time and immediately reuse it correctly in a different conjugated form which he or she has never heard, it is clear that the brain does work with rules; but irregular verbs must be processed differently. A common error for small children is to  conjugate irregular verbs as though they were regular, which is taken 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. Regular and irregular verbs are also of significance in second language acquisition, and  in particular in language teaching and formal learning, where rules such as verb paradigms are defined, and exceptions (such as irregular verbs) need to be listed and learned explicitly. The importance of irregular verbs is enhanced by the fact that they often include the most commonly used verbs in the language (including verbs such as "be" and "have" in English, their equivalents "être" and "avoir" in French, "sein" and "haben" in German, etc.). In historical linguistics the concept of irregular verbs is not so commonly referenced. Since most irregularities can be explained by processes of historical language development, these verbs  are only irregular when viewed synchronically; they often appear regular when seen in their historical context. In the study of Germanic verbs, for example, historical linguists generally distinguish between strong and weak verbs, rather than irregular and regular (although occasional irregularities still arise even in this approach). 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. By language. English. With the exception of the highly irregular verb "be", an English verb can have up to five forms: its plain form (or bare infinitive), a third person singular present tense, a past tense (or preterite), a past participle, and the "-ing" form that serves as both a present participle and gerund. The rules for the formation of the inflected parts of "regular" verbs are given in detail in the article on English verbs. In summary they are as follows: The irregular verbs of English are described and listed in the article English  irregular verbs (for a more extensive list, see List of English irregular verbs). In the case of these: Common irregular verbs. Some examples of common irregular verbs in English, other than modals, are: Other languages. For regular and irregular verbs in other languages, see the articles on the grammars of those languages. Particular articles include, for example: Some grammatical information relating to specific verbs in various languages can also be found in Wiktionary. Constructed languages. Most natural languages, to different extents, have a number of irregular verbs. Artificial auxiliary languages usually have a single regular pattern for all verbs (as  well as other parts of speech) as a matter of design, because inflectional irregularities are considered to increase the difficulty of learning and using a language. Other constructed languages, however, need not show such regularity, especially if they are designed to look similar to natural ones. The auxiliary language Interlingua has some irregular verbs, principally "esser" "to be", which has an irregular present tense form "es" "is" (instead of expected "esse"), an optional plural "son" "are", an optional irregular past tense "era" "was/were" (alongside regular "esseva"), and a unique subjunctive form "sia" (which can also function as an imperative ). Other common verbs also have irregular present tense forms, namely "vader" "to go" — "va", "ir" "to go" — "va" (also shared by the present tense of "vader"), and "haber" "to have" — "ha".