Synchronous online teaching through video-conferencing software (VCS) has become a common mode of instruction during the COVID-19 pandemic. This mode brings unique challenges for teachers that require specific professional digital competences. One such challenge is conducting formative assessment. Through the use of in-depth interviews with nine Hong Kong university teachers who had taught synchronously online for one semester, this exploratory study explored the challenges of conducting formative assessment during synchronous online lessons and the strategies teachers deploy to conduct formative assessment. The findings indicate that the key difficulties include gathering information on students' needs, checking students' understanding, giving individual feedback, and building relationships with students. To address these challenges, the teachers suggested four pedagogical strategies: (1) Combine synchronous with asynchronous approaches; (2) Utilise breakout rooms; (3) Utilise poll functions of the VCS and combine it with other online platforms; and (4) Provide time before or after lessons for informal conversations. The findings suggest that teachers need to develop specific pedagogical strategies to conduct formative assessment. This provides an initial conceptualisation of one important dimension of Professional Digital Competence, teaching using digital technologies, and one aspect of that dimension, how teachers use digital tools and strategies to conduct formative assessment.