The periodization of the historical stages of the Korean language is as follows:
Korean being a language isolate, "Proto-Korean" is not a well-defined term, referring to the language spoken in prehistoric Korea during the Bronze and Iron ages. Other theories are the Altaic and Dravido-Korean theory, but both are either discredited or fringe.
According to several linguists the linguistic homeland of proto-Korean is located somewhere in Manchuria. Later, Koreanic-speakers already present in northern Korea started to migrate further south, replacing or assimilating Japonic-speakers and likely causing the Yayoi migration. Whitman (2012) suggests that the proto-Koreans arrived in the southern part of the Korean Peninsula at around 300 BC and coexist with the descendants of the Japonic Mumun cultivators (or assimilated them). Both had influence on each other and a later founder effect diminished the internal variety of both language families.
Alexander Vovin (2015) notes that Koreanic shares some typological features with the four Paleosiberian language groups (e.g. lack of phonemic voiced stops, verb compounding, earlier ergativity), and suggests that it actually has more in common with "Paleosiberian" than with the putative Altaic group. A relation to the Japonic languages is debated but currently not accepted by most linguists.
Homer Hulbert claimed the Korean language was Ural-Altaic in his book, The History of Korea (1905). The classification of Korean as Altaic was introduced by Gustaf John Ramstedt (1928), but even within the debunked Altaic hypothesis, the position of Korean relative to Japonic is unclear. A possible Korean-Japonic grouping within Altaic has been discussed by Samuel Martin, Roy Andrew Miller and Sergei Starostin. Others, notably Vovin, interpret the affinities between Korean and Japanese as an effect caused by geographic proximity sprachbund.
Old Korean (?, ?) corresponds to the Korean language from the beginning of the Three Kingdoms of Korea to the latter part of the North-South States Period, approximately from the first to the tenth century. Use of Classical Chinese by Koreans began in the fourth century or earlier, and phonological writing in Idu script was developed by the sixth century.
It is unclear whether Old Korean was a tonal language. It is assumed that Old Korean was divided into dialects, corresponding to the three kingdoms. Of these, the Sillan language is the best attested due to the political domination of Later Silla by the seventh century.
Only some literary records of Unified Silla, changed into Goryeo text, are extant and some texts (written in their native writing system) of the Three Kingdoms period are mostly available in form of inscriptions at present. Thus, the languages of the Three Kingdoms period are generally examined through official government names and local district names.
The point at which Old Korean became Middle Korean is assessed variously by different scholars. The line is sometimes drawn during late Goryeo and sometimes around the 15th century in the early Joseon. It is usually thought that Middle Korean begins with the establishment of Goryeo and its new capital city of Kaesong, when the standard language was changed from the Silla dialect to the Goryeo dialect.
There is very little literature for research of Old Korean. The first texts in Old Korean were written using Hanja to represent the sound and grammar of the local language.
Additional information about the language is drawn from various proper nouns recorded in Korean and Chinese records, and from etymological studies of the Korean pronunciations of Chinese characters.
Various systems were used, beginning with ad hoc approaches and gradually becoming codified in the Idu script and the hyangchal system used for poetry. These were arrangements of Chinese characters to represent the language phonetically, much like the Japanese kana.
The first foreign record of Korean is the Jilin leishi, written in 1103 by a Chinese Song dynasty writer, S?n Mù . It contains several hundred items of Goryeo-era Korean vocabulary with the pronunciation indicated through the use of Chinese characters, and is thus one of the main sources for information on Early Middle Korean. From a phonological perspective however, the usefulness of this material is limited due to logographic nature of the characters.
The creation of the Hunminjeongeum ("Proper Sounds for the Instruction of the People"), the original name for Hangul, was completed in 1443 by Sejong the Great, the fourth Joseon king, and promulgated in September or October 1446.
Hunminjeongeum was an entirely new and native script for the Korean language and people. The script was initially named after the publication, but later came to be known as "Hangul". It was created so that the common people illiterate in Hanja could accurately and easily read and write the Korean language. Its supposed publication date, October 9, is now "Hangul Day" in South Korea.
In Korean wiktionary, the pronunciation of Middle Korean is represented by the Yale romanization of Korean. This is because the Revised Romanization of Korean was only designed for Modern Korean. Yale romanization of Korean places primary emphasis on showing a word's morphophonemic structure, so it does not indicate the actual pronunciation of the day.
Modern Korean (?, ?) corresponds to Korean spoken from the seventeenth century onward.
Over the decades following the Korean War and the division of Korea, North-South differences in the Korean language have developed, including variances in pronunciation, verb inflection and vocabulary.
... there are strong indications that the neighbouring Baekje state (in the southwest) was predominantly Japonic-speaking until it was linguistically Koreanized.