Could you give prospective teachers some concise advice on how and when to use feedback during a lesson (maybe in one or two quotable sentences)?

"In a nutshell, feedback is not only powerful but most variable in its impact. To maximize the power, ensure you give students “where to next” feedback (as that is what they see as powerful feedback), check often to ensure that they understand the feedback (it is as much about the receiving as the giving of feedback), and always ask yourself how you are seeking feedback about your impact (with whom, about what, and to what magnitude)” (John Hattie, personal communication, January 9, 2019).