Classification
While the majority of linguists consider Korean to be a language isolate, there have been attempts to link it with other languages in the region.
Korean is similar to the Altaic languages in lacking certain grammatical elements, including articles, fusional morphology and relative pronouns. Since Ramstedt's 1928 article, some linguists support the hypothesis that Korean can be classified as an Altaic language or as a relative of proto-Altaic. However, linguists agree today that typological resemblances cannot be used to prove genetic relatedness of languages, as these features are typologically connected and easily borrowed. Such factors of typological divergence as Middle Mongolian's exhibition of gender agreement can be used to argue that a genetic relationship is unlikely.
The hypothesis that Korean might be related to Japanese has had some supporters due to some apparent overlap in vocabulary and similar grammatical features that have been elaborated upon by such researchers as Samuel E. Martin and Roy Andrew Miller. Sergei Starostin (1991) found about 25% of potential cognates in the Japanese–Korean 100-word Swadesh list, which—if valid—would place these two languages closer together than other possible members of the Altaic family. In Korea, Korean ethnic nationalism and anti-Japanese sentiment has led to widespread dismissal of the idea of Japanese-Korean linguistic connection. A popular myth among Koreans that they are descended from "horse-riding peoples of Central Asia such as the Mongols" has led Korean scholars to continue propagating the Altaic hypothesis that has since been discredited in the West.
Other linguists, most notably Alexander Vovin, argue that the similarities are not due to any genetic relationship, but rather to a sprachbund effect and heavy borrowing especially from ancient Korean into Western Old Japanese. A good example might be Middle Korean sàm and Japanese asa "hemp". This word seems to be cognate, but while it is well-attested in Western Old Japanese and Northern Ryūkyū, in Eastern Old Japanese it only occurs in compounds, and it is only present in three subdialects of the South-Ryūkyūan dialect group. Then, the doublet wo "hemp" is attested in Western Old Japanese and Southern Ryūkyū. It is thus plausible to assume a borrowed term. (See Classification of Japonic for further details on a possible relationship.)
Among ancient languages, various closer relatives of Korean have been proposed, constituting a possible small Koreanic language family.
Read more about this topic: Korean Language