The purpose of this study is to investigate the optimal conditions for designing tasks appropriate to the elementary classroom based on the correspondence with the national curriculum, integration among four skills (listening, speaking, reading, and writing), authenticity, and interactivity. For this study, two primary English teachers volunteered to participate in the case study conducted in the spring semester of the 2012 school year. Each class observed was composed of 29 and 30 sixth graders (12-year-old learners). Data were collected through classroom observation and lesson plans. Optimality theory was used to analyze data from the lessons. From the findings, the overall ranking of constraints is Curriculum ${\gg}$ Integration ${\gg}$ Authenticity ${\gg}$ Interactivity. It is also shown that for teacher 'L', the tasks such as 'guessing game', 'photo of me', and 'role play' were appropriate to help students ask questions and give reasons for their choices. As for teacher 'C', the tasks such as 'hand spans', 'transport survey', and 'picture telling' needed to be considered in order to help students understand and write comparative sentences.