Suppletion

In linguistics and etymology, suppletion is traditionally understood as the use of one word as the inflected form of another word when the two words are not cognates. For those learning a language, suppletive forms will be seen as "irregular" or even "highly irregular". For example, go:went is a suppletive paradigm, because go and went are not etymologically related, whereas mouse:mice is irregular but not suppletive, since the two words come from the same Old English ancestor.
The term "suppletion" implies that a gap in the paradigm was filled by a form "supplied" by a different paradigm. Instances of suppletion are overwhelmingly restricted to the most commonly used lexical items in a language.

Irregularity and suppletion

An irregular paradigm is one in which the derived forms of a word cannot be deduced by simple rules from the base form. For example, someone who knows only a little English can deduce that the plural of girl is girls but cannot deduce that the plural of man is men. Language learners are often most aware of irregular verbs, but any part of speech with inflections can be irregular.
For most synchronic purposes—first-language acquisition studies, psycholinguistics, language-teaching theory—it suffices to note that these forms are irregular. However, historical linguistics seeks to explain how they came to be so and distinguishes different kinds of irregularity according to their origins.
Most irregular paradigms can be explained by phonological developments that affected one form of a word but not another. In such cases, the historical antecedents of the current forms once constituted a regular paradigm.
Historical linguistics uses the term "suppletion"
to distinguish irregularities like person:people or cow:cattle that cannot be so explained because the parts of the paradigm have not evolved out of a single form.
Hermann Osthoff coined the term "suppletion" in German in an 1899 study of the phenomenon in Indo-European languages.
Suppletion exists in many languages around the world. These languages are from various language families: Indo-Aryan, Dravidian, Semitic, Romance, etc.
For example, in Georgian, the paradigm for the verb "to come" is composed of four different roots.
Similarly, in Modern Standard Arabic, the verb ' usually uses the form ' for its imperative, and the plural of ' is '.
Some of the more archaic Indo-European languages are particularly known for suppletion. Ancient Greek, for example, has some twenty verbs with suppletive paradigms, many with three separate roots.

Example words

To go

In English, the past tense of the verb go is went, which comes from the past tense of the verb wend, archaic in this sense. See Go .
The Romance languages have a variety of suppletive forms in conjugating the verb "to go", as these first-person singular forms illustrate :
The sources of these forms, numbered in the table, are six different Latin verbs:

vādere ‘to go, proceed’,
īre ‘to go’
ambitāre ‘to go around’, also the source for Spanish and Portuguese andar ‘to walk’
ambulāre ‘to walk’, or perhaps another Latin root, a Celtic root, or a Germanic root halon or hala
fuī suppletive perfective of esse ‘to be’.
meāre ‘to go along’.

Many of the Romance languages use forms from different verbs in the present tense; for example, French has je vais ‘I go’ from vadere, but nous allons ‘we go’ from ambulare. Galician-Portuguese has a similar example: imos from ire ‘to go’ and vamos from vadere ‘we go’; the former is somewhat disused in modern Portuguese but very alive in modern Galician. Even ides, from itis second-person plural of ire, is the only form for ‘you go’ both in Galician and Portuguese.
Sometimes, the conjugations differ between dialects. For instance, the Limba Sarda Comuna standard of Sardinian supported a fully regular conjugation of andare, but other dialects like Logudorese do not. In Romansh, Rumantsch Grischun substitutes present and subjunctive forms of ir with vom and giaja in the place of mon and mondi in Sursilvan.
Similarly, the Welsh verb mynd ‘to go’ has a variety of suppletive forms such as af ‘I shall go’ and euthum ‘we went’. Irish téigh ‘to go’ also has suppletive forms: dul ‘going’ and rachaidh ‘will go’.
In Estonian, the inflected forms of the verb minema ‘to go’ were originally those of a verb cognate with the Finnish lähteä ‘to leave’, except for the passive and infinitive.

Good and bad

In Germanic, Romance, Celtic, Slavic, and Indo-Iranian languages, the comparative and superlative of the adjective "good" is suppletive; in many of these languages the adjective "bad" is also suppletive.
The comparison of "good" is also suppletive in → parem → parim and → parempi → paras.
Similarly to the Italian noted above, the English adverb form of "good" is the unrelated word "well", from Old English wel, cognate to wyllan "to wish".

Great and small

Celtic languages:
In many Slavic languages, great and small are suppletive:

Examples in languages

Albanian

In Albanian there are 14 irregular verbs divided into suppletive and non-suppletive:

Ancient Greek

had a large number of suppletive verbs. A few examples, listed by principal parts:

Bulgarian

In Bulgarian, the word is suppletive. The strict plural form,, is used only in Biblical context. In modern usage it has been replaced by the Greek loan. The counter form is suppletive as well: . For example, ; this form has no singular either.

English

In English, the complicated irregular verb to be has forms from several different roots:

be, been, being—from Old English bēon, from Proto-Germanic *beuną, from Proto-Indo-European *bʰúHt, from the root *bʰuH-.
am, is, are—from Middle English am, em, is, aren, from Old English eam, eom, is, earun, earon, from Proto-Germanic *immi, *izmi, *isti, *arun, all forms of the verb *wesaną, from Proto-Indo-European *h₁ésmi, from the root *h₁es-.
was, were—from Old English wæs, wǣre, from Proto-Germanic *was, *wēz, from the Proto-Indo-European root *h₂wes-

This verb is suppletive in most Indo-European languages, as well as in some non-Indo-European languages such as Finnish.
An incomplete suppletion exists in English with the plural of person. The regular plural persons occurs mainly in legalistic use. More commonly, the singular of the unrelated noun people is used as the plural; for example, "two people were living on a one-person salary". In its original sense of "populace, ethnic group", people is itself a singular noun with regular plural peoples.

Hungarian

The verb "to be": van, vagyok, vagy, lenni, lesz, nincs, sincs.
The verb jön has the imperative gyere.
The numeral sok has the comparative több and the superlative legtöbb.
The adverb kicsit has the comparative kevésbé and the superlative legkevésbé.
Many inflected forms of personal pronouns are formed by using the suffix as the base: nekem from -nak/-nek and -em. Even among these, the superessive form uses the root rajta instead of the suffix -on/-en/-ön.
The numerals egy, kettő have the ordinal forms első, második. However they are regular in compounds: tizenegyedik, tizenkettedik.
Irish

Several irregular Irish verbs are suppletive:

abair : derived from Old Irish as·beir, from Proto-Indo-European roots *h₁eǵʰs- and *bʰer-. However, the verbal noun rá is derived from Old Irish rád, ultimately from Proto-Indo-European *reh₂dʰ-.
bí : derived from Proto-Indo-European *bʰuH-. However, the present tense form tá is derived from Old Irish at·tá, from Proto-Celtic *ad-tāyeti, ultimately from Proto-Indo-European *steh₂-.
beir : derived from Proto-Indo-European *bʰer-. However, the past tense form rug is derived from Old Irish rouic, which is from Proto-Celtic *ɸro-ōnkeyo-, ultimately from Proto-Indo-European roots *pro- and *h₂neḱ-.
feic : derived from Old Irish aicci, from Proto-Indo-European *kʷey-. However, the past tense form chonaic is derived from Old Irish ad·condairc, ultimately from Proto-Indo-European *derḱ₂-.
téigh : derived from Old Irish téit, from Proto-Indo-European *stéygʰeti-. However, the future form rachaidh is derived from Old Irish regae, ultimately from Proto-Indo-European *h₁r̥gʰ-, while the verbal noun dul is from *h₁ludʰét.

There are several suppletive comparative and superlative forms in Irish; in addition to the ones [|listed above], there is:

fada, "long"; comparative níos faide or níos sia — fada is from Old Irish fota, from Proto-Indo-European *wasdʰos ; compare Latin vāstus, while sia is from Old Irish sír, from Proto-Celtic *sīros ; compare Welsh/Breton hir.
Japanese

In modern Japanese, the copulae だ, である and です take な to create "attributive forms" of adjectival nouns :

Irrealis 未然形	Adverbial 連用形	Conclusive 終止形	Attributive 連体形	Hypothetical 仮定形	Imperative 命令形
だろ -daro	だっ -daQ で -de に -ni	だ -da	な -na	なら -nara

The "conclusive" and "attributive" forms, だ and な, were constructed similarly, from a combination of a particle and an inflection form of the old verb あり.

で + あり → であり → であ → だ
に + ある → なる → なん → な

In modern Japanese, である simply retains the older appearance of だ, while です is a different verb that can be used as a suppleted form of だ. Multiple hypotheses have been proposed for the etymology of です, one of which is a contraction of であります:

で + あり + ます → であります → です

The basic construction of the negative form of a Japanese verb is the "irrealis" form followed by ない, which would result in such hypothetical constructions as *だらない and *であらない. However, these constructions are not used in modern Japanese, and the construction ではない is used instead. This is because *あらない, the hypothetically regular negative form of ある, is not used either, and is simply replaced with ない.

あら + ない → ない
であら + ない → ではない
だら + ない → ではない → じゃない

While the auxiliary ない causes suppletion, other auxiliaries such as ん and ありません do not necessarily.

あら + ん → あらん
あり + ませ + ん → ありません
であり + ませ + ん → でありません

For です, its historical "irrealis" form, でせ has not been attested to create a negative form. Thus, it has to borrow でありません as its negative form instead.
To express a potential meaning, as in "can do", most verbs use the "irrealis" form followed by れる or られる. する, notably has no such construction, and has to use a different verb for this meaning, できる.