This paper examines the Nominative/Accusative Case alternation observed in the object of the embedded clause in the Korean desiderative construction, -ko siphta, and proposes two different syntactic structures where the Nominative-marked object is base-generated higher in the structure than its Accusative-marked counterpart. This paper also addresses the issue of why the -ko siphta construction exhibits the proposed two different structures in the first place. Based on diachronic studies on the semantic changes of the -ko siphta construction from its 15th century to its modern form, this paper argues that from the semantic point of view the desiderative predicate siph- has lost its original meaning of ‘to think’ but it retains, from a syntactic point of view, its argument structure taking a single CP complement in addition to its newly acquired argument structure as a psych verb.