With the proposal of core competencies for the English subject, thinking quality has attracted increasing attention at home and abroad. To promote the four core competencies, the General High School English Curriculum Standards pointed out that in addition to regular teaching, teachers should organize more challenging activities, such as Project-Based Learning, to stimulate students to actively participate, enabling them to present and challenge themselves, and to cooperate better with their peers (MOE, 2020). Project-Based Learning meets the requirements of cultivating core competencies for English subjects. The General High School English Curriculum Standards (2020) proposed that PBL should be given more attention. Consequently, the new-version textbooks now include a project section to stress the significance of this learning method. Students are required to explore authentic questions profoundly and improve their abilities to analyze and solve questions, thus promoting their thinking quality. However, the implementation of Project-Based Learning is still worrying and has several problems; thus, it is of great necessity to give proper suggestions in implementing it in order to promote students' thinking quality. 

      A conceptual framework has been formed based on a thorough review of thinking quality, PBL, and cultivating thinking quality through PBL. This study aims to answer a significant question: What are the relationships between PBL and students' thinking quality development? To solve this critical question, it has been divided into three specific questions: 1) How the teachers in Y High School conducted PBL; 2) What improvement was brought to students thinking quality through implementing PBL; 3) What factors in PBL influenced students' thinking quality? The case research method has been used to answer these questions. The participatory observation method, interviews, and artificial collection were used in depth in the data collection stage to collect first-hand data and explore the entire research process. The research lasted for five weeks, and a total of 120 year seven students and six teachers from Y High School participated in this research. Before implementing the project, the researcher clarified how the PBL is conducted in the case school through interviews with six teachers, 12 past PBL products, and the class observation for two months. During the project, various data were collected to analyze students' improvements in thinking quality, including six class observations records, 17 pages of teaching materials, six lesson plans, six PPTs, video-recorded online student meetings, daily groups communication records, as well as the audio or text-version communication records between teachers and students. After implementing the PBL, the researcher collected 480 students' diaries and 17 PBL products and also interviewed 18 students. All the materials have been thoroughly analyzed in order to understand the changes in students' thinking quality and the reasons behind the changes.

This study has three core findings: 1) How the teachers in Y High School conducted PBL: teachers in Y High School strategically designed their PBL teaching plans based on the requirements of English core competencies, used differentiated English instructional teaching strategies to improve students' thinking quality; and used three kinds of assessment methods in English through the whole process to evaluate students' thinking quality. 2) What improvement was brought to students thinking quality through implementing PBL: the study found that students showed improvement in their thinking quality. Logical and critical thinking were agreed to be significantly improved by most students, while students' ideas were distinguished in creative thinking. 3) What factors in PBL influenced students' thinking quality: It was found that PBL is conducive to students' thinking quality mostly because students need to critique and revise their work and keep sustained inquiry. In addition, PBL promotes students' thinking quality through reflection, gives them opportunities to choose, and stimulates their enthusiasm to think through a product to be presented to the public. There are also many remaining challenges in thinking quality. The biggest block is students' academic burden; poor cooperation, lack of experience, and a conservative mindset are some other factors that can obstruct students' thinking quality development.

        Based on research results and discussions, implications for future PBL teaching are provided, and the limitations of this research are carefully reflected. The research findings also provide implications for future PBL. Most important, more attention needs to be attached to PBL teaching to improve students' thinking quality. In addition, practical English instructional methods should be adopted to ensure the effectiveness of PBL in promoting students' thinking quality. Meanwhile, the different ethnic backgrounds between possible audiences and the students need to be taken into consideration in English PBL. Teachers also need to be careful in selecting suitable themes and topics, especially under the background of the pandemic. Although contributions and implications are presented, the study does have limitations. This PBL was implemented during the winter vacation, thus increasing students' difficulties in cooperation. Furthermore, taking students' heavy academic load into consideration, the researcher did not include a measuring gauge, pre-test and post-test, and questionnaires in this research, and if possible, it would be better to add some quantitative data to supplement the research results.