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 if 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.

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.)^{[citation needed]}

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.