In this talk, I present preliminary findings from two experiments investigating the effects of definiteness and humanness on the use of Korean plural marker –tul and discuss their theoretical implications for formal analyses of –tul marking. While –tul is generally considered optional, previous studies suggest that definiteness makes it obligatory (Kang, 1994; Kim, 2005; Ahn & Snedeker, 2022). Additionally, human and nonhuman nouns differ with respect to –tul marking (Kang, 2007; Kim & Melchin, 2018; Park, 2020), though conflicting claims exist regarding its interaction with definiteness (Park, 2020; Choi, Yoon, & Dayal, to appear). To systematically examine these factors, I conducted two experiments—an elicited production task and a sentence pair task—manipulating definiteness and humanness. The results indicate that –tul marking is jointly influenced by these two factors in the following ways: plural-denoting, definite human nouns are most likely to be –tul marked, while indefinite nonhuman nouns are least likely, with definite nonhuman nouns and indefinite human nouns falling in between. Crucially, this pattern is gradient rather than categorical, challenging analyses that treat –tul as an obligatory functional head (Park, 2022) and instead supporting analyses that treat –tul as a (nP) modifier (Kim & Melchin, 2018). As previous studies suggest, –tul may be undergoing grammaticalization in Korean (MacDonald, 2014). If this process continues, –tul may eventually evolve into a functional head; however, the findings indicate that Korean has not yet reached this stage.