Regular verb

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 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".

Examples

In English, the verb "to talk" is regular. This is not a particularly impressive feat in English, given that there are at most only five forms of each verb (apart from to be, which has eight), but it suffices.
* Infinitive/Present tense: "talk"
* 3rd person singular present tense: "talks"
* Past tense: "talked"
* Past participle: "talked"
* Gerund/progressive (the present participle): "talking"

Compare with the verb "to write", an irregular verb.
* Infinitive/Present tense: "write"
* 3rd person singular present tense: "writes"
* Past tense: "wrote"
* Past participle: "written"
* Gerund/progressive (the present participle): "writing"

Causes of irregularity

The most irregular verbs are suppletive, that is, their paradigm is made up of what were originally parts of different verbs. That this is inevitably irregular is obvious. An example is the verb 'to be' in most European languages (see Indo-European copula).

Irregularity can be caused by a regular sound change in the history of a language which occurs only in certain phonetic environments and thus affects only parts of the verbal paradigm. An example is Grammatischer Wechsel, the consonant change between the forms was and were.

Irregularity can be caused by various forms of assimilation, which again may occur in only one part of a verb. An example is English think - thought, which displays both a vowel assimilation known as Rückumlaut and an entirely separate consonant assimilation known as the Germanic spirant law.

Irregularity can be caused by a shift of stress. For example, Latin verbs usually have their stress on the penultimate syllable. Since most present tense forms have a single-syllable inflection, this means the stress is on the stem (pósso, 'I can'), but the first and second persons plural have a two-syllable inflection, which meant the stress was on the inflection (possémus, 'we can'). In French a process of levelling means that stress is always on the stem. The vowel peculiarities typical of the nous and vous forms of French irregular verbs result from the fact that only these parts experienced a shift of stress: je peux, nous pouvons.

Sometimes a regular system may fall into relative disuse, so that its remnants are seen as irregular. An example is the Germanic strong verb, such as English speak. (But beware: the weak-strong distinction in English and other Germanic languages does not co-incide with the regular-irregular distinction!)

Sometimes a perfect stem may come to be used as a present. (For example, the past tense of a verb meaning "to see" may come to be the present tense of a verb meaning "to know" - I know something if I have seen it.) The resulting verb then has past-tense inflections throughout its present tense, and may have gained a new (regular or irregular) past tense. Verbs of this sort in English are called preterite-presents; examples are can, may, must.

Almost all irregularities in verbs can be explained as part of the natural historical development of language, and grow out of some originally regular phenomenon.