This paper examines the syntactic structure of the -ko siphta phrase. The discussion focuses on the substitution phenomena, argument realization, and case assignment. The results reveal that when NP-lul is present in the -ko siphta phrase, the main verb chooses the preceding argument ahead of the trailing auxiliary verb. Siphta does not affect the realization of the argument of the embedded clause. Since the main verb loses [+transitivity] and acquires [+stative] in combination with siphta and since the main verb is prevented from assigning accusative case, the syntactic structure of -ko siphta can only be seen as a complex sentence structure. The third-person subject cannot be an agent, while the first-person subject has a dual status of both an experiencer and an agent in the -ko siphta phrase. The presumed simple sentence structure of -ko siphta, with the auxiliary and main verbs formed as one unit, would require different components in front of this unit. It is not universal and can be seen as a derivative of the base structure of -ko siphta.