On the voyage into explori ng a research question: Where is an interactional interface to interrelate a focus on form to a specific task in designing a task-based syllabus?, this study identifies three problems, i.e., the learnability, the performability, and the interactionability problem from the reviews of the research on task-based sy llabuses (Ellis, 2003; Prabhu, 1987; Snow, Met, & Genesee, 1989) to be settled in designing a task-based syllabus. In order to search for a poss ible solution to these problems, it proposes an interactive approach to a task-based sy llabus called the Ability-Task Function (ATF) Model. The ATF constitutes a linear function, 'y=ax+b', in which 'y ' represents the levels of language abi lities on the Parallel Developmental Sequence (Kim & K won, 2007) and 'x' the hierarchies of task performances on the target language use domain continuum (Kim, 2006c). Hence, the vertical y axis (i.e., the levels of language abilities) and the horizontal x ax is (i.e., the hierarchies of task performances) comprise the ATF Coordinate or Map on which we are able to explore a possible answer to the three problems. Finally, this study demonstrates how the A TF fades the three problems out, and hence comes to a conclusion: The A TF is, in a sense, a kind of metric by which not only we are able to plot, analyze, and evaluate all the pedagogic tasks on the line (i.e., y=ax+b, a= l, b=O), but also select and sequence them along the line on which they are all in direct proportion of the levels of language abilities to the hierarchies of task performances. Thus, the ATF Model can provide a conceptual framework for designing an interactive abilitydifferentiated task-based syllabus.