1 Introduction

In the past decade, ESP courses in domestic universities have received attention, but the shortage of teaching staff has become prominent. The transformation of EGP teachers into ESP teachers has become the main force, and their PCK and ability are the key to successful transformation. Existing research mostly focuses on identity recognition and professional development issues during transformation, while empirical research on the development process of PCK after transformation is relatively scarce. This study aims to explore the main contradictions of EGP teachers in teaching practice and how their resolution actions promote the development of PCK in the context of ESP transformation.

2 Theoretical Framework

The activity theory analysis framework emphasizes the impact of social activities on human learning and cognitive development. Engestr ö m proposes that learning activities are the collision and conflict resolution process of "multiple voices" within and between activity systems. The dilemma and tension of the teaching profession make the activity theory framework an effective tool for analyzing teachers' teaching practices and professional development activities. Subject Teaching Knowledge (PCK) is the process by which a teacher transforms subject knowledge into a form that students can understand, including course knowledge, knowledge that students comprehend, knowledge of teaching strategies, and specific subject teaching objectives. The development characteristics of PCK range from static frameworks to dynamic development models, emphasizing the importance of the interaction between teachers' personal factors and the environment. This study constructed a conceptual framework of PCK for ESP teachers in medical colleges, integrating teaching knowledge, language, and subject knowledge, and expanding teachers' teaching situational knowledge and students' knowledge, as well as students' needs analysis ability elements.

3 Research Design

This chapter focuses on the ESP teaching practice of two medical college English teachers, exploring the contradictions and conflicts encountered by teachers during the transformation process from the perspective of activity theory, and how they resolve these contradictions and develop their subject teaching knowledge (PCK). The research subjects are Professor Lin and Professor Xiao, who hold master's and doctoral degrees in English language and literature, respectively, and mainly teach medical English courses. Data collection includes in-depth interviews, classroom observations, instructional design and lesson plans, as well as course group preparation and discussion to ensure the comprehensiveness and credibility of the research. The data analysis adopts the grounded theory method to encode and summarize the collected data topics, and compare and analyze the changes in PCK before and after teacher transformation. The study also invited a second author to conduct thematic extraction and summarization of data to improve the reliability of the research. Through cross case studies, the mediating regulatory tools and their developmental characteristics of PCK construction by teachers after ESP transformation were revealed.

4 Research Results

From the perspective of activity theory, the main conflicts and contradictions experienced by ESP transition teachers in the development of subject teaching knowledge were studied. Through Engestr ö m's activity system conflict hierarchy analysis framework, it was found that teachers face four levels of conflicts during the transformation process: primary conflicts involve identity confusion and ability questioning; The second level of contradiction is the internal conflict caused by changes in teaching content and objectives; The third level contradiction is the contradiction between the existing teaching experience and the new teaching mode; The fourth level contradiction is the conflict between teaching investment and scientific research identification. These contradictions are seen as catalysts for teacher reform rather than obstacles.

5 Discussions

This study analyzes the contradictions and conflicts encountered by ESP transition teachers in the new teaching system from the perspective of activity theory, and how these contradictions promote the development of teachers' subject specific teaching knowledge (PCK) through extended learning. Research has found that teachers can alleviate identity anxiety and improve teaching effectiveness by resolving primary conflicts and actively participating in teaching activities. The development of teachers' PCK has non-linear and personalized characteristics, influenced by both personal factors and external environmental factors. The study emphasizes the importance of teacher initiative in resolving conflicts and suggests that ESP teacher education should focus on teachers' subject knowledge learning, initiative, and professional community building to promote the development of teacher PCK.

6 Conclusion

Subject teaching knowledge is the core of ESP teachers' professional development. This study analyzes the contradictions and conflicts encountered by transitional teachers in practical activities from the perspective of activity theory, and how these contradictions promote teachers to reconstruct teaching goals, models, and identities, thereby promoting the development of PCK. The study emphasizes the moderating role of individual teacher initiative and community in the development of PCK, and provides a basis for ESP teacher education policies. The limitations of the study lie in the representativeness of the sample size and disciplinary attributes. It is recommended that future research expand the scope of the research objects and disciplines, and explore in depth the development of ESP teachers' PCK and professional sustainability.