Engagement is positively correlated with many educational outcomes. However, engaging learners in online learning is often challenging. In this study, a conceptual framework comprising five interrelated factors (instructors, learners, content, technology, and environments) was proposed. The purpose of the study was to explore how learners could be engaged by following the conceptual framework in synchronous online learning. Fifty-five adult learners took part in the study. Specific strategies were applied in four classes. A survey with 38 five-point Likert scale items and an open-ended question was administered. Quantitative and qualitative data were collected and analysed. Results showed that instructors, learners, and content were the core factors affecting learners' engagement. Comparatively, the learners' engagement was less affected by the factors of technology and environments. Results further showed that useful strategies to engage learners included providing opportunities for instructors and peers to interact frequently; having relevant content that could apply to practice; involving interactive activities like group discussions and peer feedback; and having informal conversations with individual learners. This study suggests that future studies can investigate facilitating synchronous online discussions, establishing social connectedness, and using technology to monitor learners' engagement automatically.