The purpose of this study is to specifically clarify why the interrogative final endings that form a noun clause equivalents appears only in interrogative final endings rather than in other final endings. Additionally, the non-final function of interrogative final endings was explored, and the necessity of discussing certain interrogative final endings as 'Interrogative final endings of noun clause function' was reconsidered. The study aimed to understand why endings that function as nominal clauses appear only in interrogative final endings and why only specific interrogative final endings can be used as nominal clause functions. Furthermore, it discussed why other final endings, despite their ability to be used for other non-final functions, cannot be used as non-final functions of nominal clauses, and why not all interrogative final endings can be used as nominal clause functions. The discussion also covered only the nominal clause structures formed by interrogative final endings can express the meaning area of 'inquiry about an unverified fact,' which existing Korean nominal clauses such as '-(으)ㅁ', '-기’ nominal clauses and '것' noun clause equivalents cannot convey. Finally, it discussed that due to the nominal characteristic of requiring a referent, interrogative final endings like '-느냐', '-는가', '-는지', and '-을까' can be used as nominal clause functions, while other interrogative final endings cannot.