This paper is aimed at considering the 'n'(ㄴ) insertion in Korean history, which is being used in modern national language. Though there have been a lot of research on the 'n'(ㄴ) insertion from the early years with various views, so far, the substantive study on Korean history to figure it out has not been found.
Referring to the 'n'(ㄴ) insertion of modern national language, these words '첫여름, 홑이불, 나뭇잎, 옛일‘ as marked in general are historically intended as examples. In the 15th century, the each form of these words, ‘첫여름, 홑이불, 나뭇잎, 옛일’, was marked as '녀름, 니블, 닢, 일‘ respectively, which likewise, lasted over the 17th century. In the 18th century, there existed two forms of each word as ’녀름/여름, 니블/이블, 닢/잎, 일‘, which were used until the 19th century.
The compounds of these words (including derivative) in modern national language are '첫여름, 홑이불, 나뭇잎, 옛일‘. However, in the 15th century, the each form of these words was '(x), 홋니블, 나못닢, 녯일’ (cf. The compound of ’여름‘ was not verified and ‘녯, 옛’ of '녯일‘ will be mentioned in the text.) and they showed until the 17th century. In the 18th century, the each form of these words was '첫녀름/쳣여, 홋니블, 나못닢, 녯일(옛일)/옛닐‘ respectively and was changed to '쳣여럼 , 홋니블/홋이블, 나못닢, 녯일(옛일)/옛닐(녯닐)' in the 19th century.
In modern national language, they are '여름, 이불, 잎, 일‘ that omit the letter 'n''. Considering the examples above, the 'n' before the 'i or j' of an initial sound was omitted a little ahead of the 18th century. Seeing these examples, the 'n' insertion is presumed to be shown in the 18th century. The basic environment of the 'n' insertion is that the first syllable of the following letter begins with 'i or j' on the boundary of the morpheme in compounds.