How did the Korean language borrow words from Classical Chinese?

The Korean language has historically incorporated a significant number of loanwords from Classical Chinese, which account for most technical terminology and approximately 10 percent of basic nouns, such as san (mountain) and kang (river).1

This linguistic integration was facilitated by centuries of close cultural contact. For much of Korean history, the upper classes were bilingual in a specific sense: while they spoke Korean, they utilized Chinese for writing, literature, and official administration. Because Korea lacked a native writing system until the invention of Hangul in the 15th century, scholars and officials relied on Chinese characters to record their language.23

To bridge the gap between the two languages, Koreans developed various transcription systems, such as idu, hyangch’al, and kugyŏl, which used Chinese characters to represent Korean grammatical elements, particles, and sentence structures. While these borrowed words were traditionally written using Chinese characters, modern usage increasingly favors writing them in the native Hangul alphabet, reserving characters primarily as aids for clarifying technical terms.214