The pre-final ending in Korean of the 15th century, ``-O-``, takes part in composing NP-embedded sentences by putting in front of ``-n``, ``-l``, and ``-m``, but ``-O-`` appears in connective suffix and final endings as well. According to the standard and basis, there is a list of ending forms involved with ``-O-`` and allomorph. Various scholars stated that when ``-O-`` appears with ``-si-``, ``-deo-``, and ``-geo-``, it is written into ``-sha-``, ``-da-``, and ``-ga-``, but there is still not enough explanation about these allomorphs, especially about ``-gua-`` and ``-ro-``. In middle Korean, ``-gua-`` appears in ``-guara``, ``-guaida``, ``-guandai``, ``-guandyeo``, ``-guadyeo``, and ``-guadai``. Because ``-gua-`` appears relatively limitedly, there is difficulty in understanding its function. ``-gua-`` may be explained as a union of ``-geo-`` and ‘-O-’ according to existing opinions, but it doesn``t appear very convincing. This study set standards for problems and direction of studies on ``-gua-``. (1) Do ``-gua-`` sentence structures have something in common? Is there semantic similarity or syntactic similarity? If same function cannot be found, the form needs to be ruled out. (2) Is it possible to analyze ``-gua-`` of the 15th century? It was identified that ``-gua-`` of the 15 century is impossible to analyze. (3) If it si possible to analyze, is it ``-geo-``+``-O-``? An explanation that the result of a combination of ``-geo-`` and ``-O-`` being ``-ga-``, ``-gua-`` sounds a little unconvincing. (4) Then, can a hypothesis that it is a combination with a prefinal ending ``-go-`` not ``-geo-`` be proposed? There is ``-go-`` in old Korean language, but it may not be proved by limited literature. It is because compositions that correspond to ``-gua-`` and ``-ga-`` did not appear in Seokdokkugyol.