Yukjin also makes up a large component of Koryo-mar, the forms of Korean spoken by the descendents of people deported from the Russian Far East to Central Asia by Stalin.

There have been many attempts to link Koreanic with other language families, most often with Tungusic or Japonic, but no genetic relationship has been conclusively demonstrated.

[6][7] Dialects differ in palatalization and the reflexes of Middle Korean accent, vowels, voiced fricatives, word-medial /k/ and word-initial /l/ and /n/.

[12] During this period, Korean absorbed a huge number of Chinese loanwords, affecting all aspects of the language.

[26] Some 250,000 Koreans lived in the area in the 1930s, when Stalin had them forcibly deported to Soviet Central Asia, particularly Uzbekistan and Kazakhstan.

[30] The form of Korean spoken in Japan also shows the influence of Japanese, for example in a reduced vowel system and some grammatical simplification.

[32] The speech of Jeju Island is not mutually intelligible with standard Korean, suggesting that it should be treated as a separate language.

[1][37] When King Sejong drove the Jurchen from what is now the northernmost part of North Hamgyong Province in 1434, he established six garrisons (Yukchin) in the bend of the Tumen River – Kyŏnghŭng, Kyŏngwŏn, Onsŏng, Chongsŏng, Hoeryŏng and Puryŏng – populated by immigrants from southeastern Korea.

Modern varieties show limited variation, most of which can be treated as derived from Late Middle Korean (15th century).

[44] Some authors have proposed that Late Middle Korean [jə] ⟨ㅕ⟩ reflects an eighth Proto-Korean vowel, based on its high frequency and an analysis of tongue root harmony.

[90] Korean also resembles Japonic and Ainu in having a single liquid consonant, while its continental neighbours tend to distinguish /l/ and /r/.

[92] A similar pitch accent is found in Japonic and Ainu languages, but not Tungusic, Mongolic or Turkic.

[95] There have been several attempts to link Korean with other language families, with the most common being the controversial "Altaic" group (Tungusic, Mongolic and Turkic) and Japonic.

[109][110] However, no evidence of these migrations has been found, and archaeologists now believe that the Korean peninsula and adjacent areas of eastern Manchuria have been continuously occupied since the Late Pleistocene.

[113][114] Moreover, most comparativists no longer accept the core Altaic family itself, even without Korean, believing most of the commonalities to be the result of prolonged contact.

[93][115] The shared features turned out to be rather common among languages across the world, and typology is no longer considered evidence of a genetic relationship.

[122] Scholars outside of Korea have given greater attention to possible links with Japonic, which were first investigated by William George Aston in 1879.

[124] Samuel Martin, John Whitman and others have proposed hundreds of possible cognates, with sound correspondences.

[127][128] However, Koreanic and Japonic have a long history of interaction, which may explain their grammatical similarities and makes it difficult to distinguish inherited cognates from ancient loanwords.

[129][130] Most linguists studying the Japonic family believe that it was brought to the Japanese archipelago from the Korean peninsula around 700–300 BC by wet-rice farmers of the Yayoi culture.

[131][125] Placename glosses in the Samguk sagi and other evidence suggest that Japonic languages persisted in central and southwestern parts of the peninsula into the early centuries of the common era.

[142] There is a tendency in Korea to assume that all languages formerly spoken on the peninsula were early forms of Korean, but the evidence indicates much greater linguistic variety in the past.

[143] Chinese histories provide the only contemporaneous descriptions of peoples of the Korean peninsula and eastern Manchuria in the early centuries of the common era.

[150] To the south lay the Samhan ('three Han'), Mahan, Byeonhan and Jinhan, who were described in quite different terms from Buyeo and Goguryeo.

[153][154] Based on this text, Lee Ki-Moon divided the languages spoken on the Korean peninsula at that time into Puyŏ and Han groups.

[155] Lee originally proposed that these were two branches of a Koreanic language family, a view that was widely adopted by scholars in Korea.

The issue is politically charged in Korea, with scholars who point out differences being accused by nationalists of trying to "divide the homeland".

[172] The most widely cited evidence for Goguryeo is chapter 37 of the Samguk sagi, a history of the Three Kingdoms period written in Classical Chinese and compiled in 1145 from earlier records that are no longer extant.

[173] This chapter surveys the part of Goguryeo annexed by Silla, listing pronunciations and meanings of placenames, from which a vocabulary of 80 to 100 words has been extracted.

[182] It is generally agreed that these glosses demonstrate that Japonic languages were once spoken in part of the Korean peninsula, but there is no consensus on the identity of the speakers.