Tense, aspect and epistemic modality belong to the background information of sentences and have the effect of transforming virtual events into actual events. Linguistic typology studies show that the development of future tense into epistemic modality is a common feature of human languages. The future tense markers "- 겠 -" and "Yao" in Korean and Chinese language have the conceptual meanings of "speculation" and "intention", which are used to describe the future events. The description of the future events can be divided into objectively inevitable events and subjectively inevitable or possible events. For these two cases, due to the uncertainty of future events, both of them show strong subjectivity. Therefore, they are closely related to epistemic modality. Epistemic modality can be divided into three categories according to the speaker's certainty of subjective evaluation: inevitability, possibility and virtuality. This study aims to find out the similarities and differences of "- 겠 -" and "Yao" in syntax, semantics and pragmatics, and explain the cognitive mechanism behind the differences by comparing the degree of grammaticalization and subjectification.