This study concerns the meaning generated from a particle -nun in Korean combined with a prosodic accent and the characteristic pragmatic behavior of the sentences containing it. In my previous research, I have argued that the alleged Topic marker -nun generates an existential presupposition of a contrastive alternative. In this paper, I will illustrate how the existential presupposition enables the utterances containing -nun obtain different felicity values from the canonical case marked ones in various pragmatic contexts. Speaker`s presupposition and common ground (Stalnaker1972, 1974) and accommodation (Lewis1979) will be adopted to explain the felicity differences between -nun and a case marker in the aspect of the amount of information obtained from the existential presupposition, not in the aspect of information structure differences.