This study outlines the construction of a tool to enhance the competence of foreign language teachers in estimating the effectiveness of language-learning tasks. Five assumptions basic to second-language acquisition are crucial to the design of the tool. Subsequently, drawing on insights from cognitive psychology about information-processing, a category system of learning activities is developed. This description holds clues to the nature of the mental actions carried out in the working memory. After presenting two tasks to demonstrate the tool, its contribution to the enhancement of the teacher's competence in estimating the learning impact of language-learning tasks is discussed.