Yes and no are two words for expressing the affirmative and the negative respectively in modern English.
English originally used a four-form system up to and including Early Middle English but Modern English has reduced this to a two-form system consisting of just 'yes' and 'no'. Some languages do not answer yes-no questions with single words meaning 'yes' or 'no'. Welsh and Finnish are among several languages that typically employ echo answers (repeating the verb with either an affirmative or negative form) rather than using words for 'yes' and 'no', though both languages do also have words broadly similar to 'yes' and 'no'. Other languages have systems named two-form, three-form, and four-form systems, depending on how many words for yes and no they employ. Some languages, such as Latin, have no yes-no word systems.
In linguistics, morphology is the identification, analysis and description of the structure of a given language's morphemes and other linguistic units, such as root words, affixes, parts of speech, intonation/stress, or implied context (words in a lexicon are the subject matter of lexicology). Morphological typology represents a method for classifying languages according to the ways by which morphemes are used in a language—from the analytic that use only isolated morphemes, through the agglutinative ("stuck-together") and fusional languages that use bound morphemes (affixes), up to the polysynthetic, which compress many separate morphemes into single words. (One of the definitions for Morphology).]clarification needed[
While words are generally accepted as being (with clitics) the smallest units of syntax, it is clear that in most languages, if not all, words can be related to other words by rules (grammars). For example, English speakers recognize that the words dog and dogs are closely related—differentiated only by the plurality morpheme "-s", which is only found bound to nouns, and is never separate. Speakers of English (a fusional language) recognize these relations from their tacit knowledge of the rules of word formation in English. They infer intuitively that dog is to dogs as cat is to cats; similarly, dog is to dog catcher as dish is to dishwasher, in one sense. The rules understood by the speaker reflect specific patterns, or regularities, in the way words are formed from smaller units and how those smaller units interact in speech. In this way, morphology is the branch of linguistics that studies patterns of word formation within and across languages, and attempts to formulate rules that model the knowledge of the speakers of those languages.
Most English words ending in -us, particularly those derived from Latin, replace the -us suffix with -i to form plurals. This is irregular, however: some words that end in -us do not pluralize with -i. Sometimes this is because they are not Latin words, and sometimes due to habit (e.g. campus, plural campuses). Conversely, some non-Latin words ending in -us or Latin words that would not have pluralized with -i in Latin are given an -i ending in English.
In between these two extremes are words that, despite not supporting a Latin plural on etymological grounds, are nonetheless widely pluralized with -i and as such are not immediately heard as incorrect by a substantial number of native speakers (e.g. octopi as a plural for octopus). The question of whether or not these alternative plural forms can be considered incorrect or not touches on the on-going prescriptivism versus descriptivism debate in linguistics and language education.
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