This is the first book in the series Mother-tongue Education in Hong Kong . Having collected some 8,000 examination questions at junior secondary levels from more than 20 secondary schools that had switched to mother-tongue teaching in 1998, the Support Centre for Teachers Using Chinese as the Medium of Instruction (CMI Centre) of the University of Hong Kong analysed and studied the errors in examination questions from the perspective of inter-language theory and transformation. The result of the study shows the level of written Chinese proficiency of junior secondary school teachers who had just switched to mother-tongue teaching in 1998. The book aims to identify types and causes of errors in setting examination questions and ways to correct and avoid them. It then explores language issues arising from the inter-language influence between Cantonese and Putonghua as well as the switch to Chinese from English. As the book analyses materials collected from schools using meticulous research procedures, it is valuable in terms of both academic and practical authenticity. The book can be used as a reference book for teacher training to enhance trainees’ written Chinese proficiency, and teachers can also use it as a reference for self study.