This study investigates the grammatical forms appearing in the Korean Old Vernacular Letters of the Archives of So Clan in Tsusima Island. Focusing on the frequently appearing forms, this study closely examines and elucidates the particularities found in these documents from a historical linguistic point of view. If we compare these documents to others composed at a similar time, we see that while they include mixed use of Chinese and Korean orthographic styles, they nonetheless should be judged as faithfully reflecting the actual language of that time as much as the documents are written using primarily in Korean orthography. For example, while the subjective case particle ``ga`` appears in this text, the conservative forms which frequently appear in the vernacular Korean translations of Chinese texts of that time (for example the subjective case particle ``y,`` the connective ending ``-od^y``) are almost entirely not observed. Notably, if we compare the documents with the books that were used for learning Japanese language, especially the Ineo Daebang ``Great Guide to a Neighboring Language,`` we can see commonalities for example the subjective case particle ``ga`` and ``gyeosyeo,`` the second person addressive expression ``gong 公,`` etc., but on the other hand, when compared to other vernacular letters of the time, several clear differences emerge; for example the dative case particle ``ge,`` the honorific subjective case particle ``gyeosyeo,`` the connective ending ``-eudoe,`` and the first person referential expression, ``bok 僕``. In the following, I will take up the task of where these particularities arise from.