With the spread of the coronavirus disease 2019 (COVID-19), online education has been increasingly adopted globally. However, whether the online teaching approach is effective for students' learning engagement and motivation is still an open question. To improve students' learning engagement and motivation to minimize students' indulgent in procrastination and plagiarism behavior, an experimental case study on forum-based online teaching was carried out. Results showed that compared to traditional class teaching, the forum-based online teaching effectively improved students' learning engagement and motivation as well as reduced procrastination and plagiarism. Although some students are emotionally resistant to this new forum-based online teaching method, most of the students believed their presentation and other academic skills could be improved through forum-based online teaching. Moreover, students generally accepted moderate levels of peer pressure and competition that were created by the forum-based learning process. Therefore, forum-based online teaching can be considered as a useful complementary approach to traditional class teaching. The implications of this study include that breaking a final "term paper" into multiple small online submissions helps students proactively complete homework assignments and avoids plagiarism. Moreover, educators integrate theories into students' life experiences through online teaching forums, which also improve student learning engagement and motivation.