With the vigorous development of the Internet, e-learning systems have become more and more popular. Currently, to solve the issue of sharing and reusing teaching materials in different e-learning systems, several standard formats, including SCORM, IMS, LOM, AICC etc., have been proposed by international organizations. Among these international standards, the Sharable Content Object Reference Model (SCORM) has become the most popular standard in recent years. In the SCORM standard, Sequencing and Navigation (SN), which is based on the concepts of learning activities, defines the course sequencing behavior, which controls the sequencing, selecting and delivering of the course, and organizes the content into a hierarchical structure, namely, an Activity Tree (AT). For a large-scale learning activity, the Activity Tree will become too complex to manage and reuse, as will its sequencing behavior rules. Moreover, the lack of inter-relations among Activity Trees also makes reusing and integrating them hard. This implies that the scalability and flexibility of an adaptive learning system will be limited. Therefore, how to create, represent and maintain an AT and associated sequencing definitions is our concern. In addition, for a personalized learning environment, how to extend the structure of an AT with Pedagogical Theory has also become an important issue. Therefore, in this paper, we extend and modularize the structure of an AT by applying Pedagogical Theory and the concept of the Object Oriented Methodology, respectively. Thus, we first propose a novel model, the Instructional Activity Model (IAM), which is composed of related AT nodes. Each AT node in IAM is modularized as a learning unit with inter-relations and specific attributes, which can be easily managed, reused, and integrated. We also propose an AT Selection algorithm with a pedagogical strategy for traversing IAM in order to generate dynamic learning content for the learner. IAM with its scalability and flexibility can apply appropriate pedagogical theories to meet specific needs by means of extension scheme. Finally, we propose a systematic approach to fast and easily construction of IAM using traditional course resources.