This paper aims to clarify the differences between TO complement clauses (attributive form + bound noun TO / 'thing') and -WOM complement clauses (nominalized clauses). Based on an analysis of two Late Middle Korean texts, I demonstrate that TO complement clauses primarily appear as the complement of predicates dealing with knowledge and perception, especially those containing the verb al- 'to know'. On the other hand, -WOM complement clauses have no restriction with respect to complement-taking predicates. I argue that this is because TO complement clauses can express tense, aspect, and mood by using the attributive form of the verb preceding the bound noun, while -WOM complement clauses do not permit this range of possibilities. In this way, speakers or writers can use TO complement clauses to express factivity (or a lack thereof) in the proposition of the complement clause. Thus, factive complement-taking predicates such as al- ('to know') tend to co-occur with TO complement clauses.