Yes and no

Yes and no, or similar word pairs, are expressions of the affirmative and the negative, respectively, in several languages, including English. Some languages make a distinction between answers to affirmative versus negative questions and may have three-form or four-form systems. English originally used a four-form system up to and including Early Middle English. Modern English uses a two-form system consisting of yes and no. It exists in many facets of communication, such as: eye blink communication, head movements, Morse code, and sign language. Some languages, such as Latin, do not have yes–''no word systems.
Answering a "yes or no" question with single words meaning yes or no is by no means universal. About half the world's languages typically employ an echo response: repeating the verb in the question in an affirmative or a negative form. Some of these also have optional words for yes and no, like Hungarian, Russian, and Portuguese. Others simply do not have designated yes and no words, like Welsh, Irish, Latin, Thai, and Chinese. Echo responses avoid the issue of what an unadorned yes means in response to a negative question. Yes and no can be used as responses to a variety of situationsbut are better suited in response to simple questions. While a yes response to the question "You don't like strawberries?" is ambiguous in English, the Welsh response ydw has no ambiguity.
The words yes and no are not easily classified into any of the conventional parts of speech. Sometimes they are classified as interjections. They are sometimes classified as a part of speech in their own right, sentence words, or pro-sentences, although that category contains more than yes and no, and not all linguists include them in their lists of sentence words. Yes and no'' are usually considered adverbs in dictionaries, though some uses qualify as nouns. Sentences consisting solely of one of these two words are classified as minor sentences.

In English

Classification

Although sometimes classified as interjections, these words do not express emotion or act as calls for attention; they are not adverbs because they do not qualify any verb, adjective, or adverb. They are sometimes classified as a part of speech in their own right: sentence words or word sentences.
This is the position of Otto Jespersen, who states that Yes' and 'No'... are to all intents and purposes sentences just as much as the most delicately balanced sentences ever uttered by Demosthenes or penned by Samuel Johnson."
Georg von der Gabelentz, Henry Sweet, and Philipp Wegener have all written on the subject of sentence words. Both Sweet and Wegener include yes and no in this category, with Sweet treating them separately from both imperatives and interjections, although Gabelentz does not.
Watts classifies yes and no as grammatical particles, in particular response particles. He also notes their relationship to the interjections oh and ah, which is that the interjections can precede yes and no but not follow them. Oh as an interjection expresses surprise, but in the combined forms oh yes and oh no merely acts as an intensifier; but ah in the combined forms ah yes and ah no retains its stand-alone meaning, of focusing upon the previous speaker's or writer's last statement. The forms *yes oh, *yes ah, *no oh, and *no ah are grammatically ill-formed. Aijmer similarly categorizes the yes and no as response signals or reaction signals.
Felix Ameka classifies these two words in different ways according to the context. When used as back-channel items, he classifies them as interjections; but when they are used as the responses to a yes–no question, he classifies them as formulaic words. The distinction between an interjection and a formula is, in Ameka's view, that the former does not have an addressee, whereas the latter does. The yes or no in response to the question is addressed at the interrogator, whereas yes or no used as a back-channel item is a feedback usage, an utterance that is said to oneself. However, Sorjonen criticizes this analysis as lacking empirical work on the other usages of these words, in addition to interjections and feedback uses.
Bloomfield and Hockett classify the words, when used to answer yes–no questions, as special completive interjections. They classify sentences comprising solely one of these two words as minor sentences.
Sweet classifies the words in several ways. They are sentence-modifying adverbs, adverbs that act as modifiers to an entire sentence. They are also sentence words, when standing alone. They may, as question responses, also be absolute forms that correspond to what would otherwise be the not in a negated echo response. For example, a "No." in response to the question "Is he here?" is equivalent to the echo response "He is not here." Sweet observes that there is no correspondence with a simple yes in the latter situation, although the sentence-word "Certainly." provides an absolute form of an emphatic echo response "He is certainly here." Many other adverbs can also be used as sentence words in this way.
Unlike yes, no can also be an adverb of degree, applying to adjectives solely in the comparative, and an adjective when applied to nouns.
Grammarians of other languages have created further, similar, special classifications for these types of words. Tesnière classifies the French oui and non as phrasillons logiques. Fonagy observes that such a classification may be partly justified for the former two, but suggests that pragmatic holophrases is more appropriate.

The Early English four-form system

While Modern English has a two-form system of yes and no for affirmatives and negatives, earlier forms of English had a four-form system, comprising the words yea, nay, yes, and no. Yes contradicts a negatively formulated question, No affirms it; Yea affirms a positively formulated question, Nay contradicts it.

Will they not go? — Yes, they will.
Will they not go? — No, they will not.
Will they go? — Yea, they will.
Will they go? — Nay, they will not.

This is illustrated by the following passage from Much Ado about Nothing:
Benedick's answer of yea is a correct application of the rule, but as observed by W. A. Wright "Shakespeare does not always observe this rule, and even in the earliest times the usage appears not to have been consistent." Furness gives as an example the following, where Hermia's answer should, in following the rule, have been yes:
This subtle grammatical feature of Early Modern English is recorded by Sir Thomas More in his critique of William Tyndale's translation of the New Testament into Early Modern English, which was then quoted as an authority by later scholars:
In fact, More's exemplification of the rule actually contradicts his statement of what the rule is. This went unnoticed by scholars such as Horne Tooke, Robert Gordon Latham, and Trench, and was first pointed out by George Perkins Marsh in his Century Dictionary, where he corrects More's incorrect statement of the first rule, "No aunswereth the question framed by the affirmative.", to read nay. That even More got the rule wrong, even while himself dressing down Tyndale for getting it wrong, is seen by Furness as evidence that the four word system was "too subtle a distinction for practice".
Marsh found no evidence of a four-form system in Mœso-Gothic, although he reported finding "traces" in Old English. He observed that in the Anglo-Saxon Gospels,

positively phrased questions are answered positively with
and negatively with , , and meaning 'not I' ;
while negatively phrased questions are answered positively with
and negatively for example with, meaning 'no one'.

Marsh calls this four-form system of Early Modern English a "needless subtlety". Tooke called it a "ridiculous distinction", with Marsh concluding that Tooke believed Thomas More to have simply made this rule up and observing that Tooke is not alone in his disbelief of More. Marsh, however, points out that the distinction both existed and was generally and fairly uniformly observed in Early Modern English from the time of Chaucer to the time of Tyndale. But after the time of Tyndale, the four-form system was rapidly replaced by the modern two-form system. The Oxford English Dictionary says the four-form system "was usually considered to be... proper..." until about 1600, with citations from Old English and without any indication that the system had not yet started then.

Colloquial forms

Non-verbal

Linguist James R. Hurford notes that in many English dialects "there are colloquial equivalents of Yes and No made with nasal sounds interrupted by a voiceless, breathy h-like interval or by a glottal stop " and that these interjections are transcribed into writing as ' or '. These forms are particularly useful for speakers who are at a given time unable to articulate the actual words yes and no. The use of short vocalizations like uh-huh, mm-hmm, and yeah are examples of non-verbal communication, and in particular the practice of backchanneling.
Art historian Robert Farris Thompson has posited that mm-hmm may be a loanword from a West African language that entered the English vernacular from the speech of enslaved Africans; linguist Lev Michael, however, says that this proposed origin is implausible, and linguist Roslyn Burns states that the origin of the term is difficult to confirm.