This study investigates patterns exhibited by pre-service teachers (PSTs) while practicing feedback in response to students' solutions on a procedure-based mathematics assessment. First, we developed an analytical framework for understanding mathematics PSTs' written feedback. Second, we looked into how a learning module on a multimedia platform influenced PSTs' feedback, and identified the ways in which PSTs subsequently revised their initial written feedback. From this analysis, we derived an operational list of emergent patterns of PSTs' written and content-specific feedback. Our findings suggest that PSTs, in general, are likely to improve their written feedback when they engage in reviewing student work, and have the chance to write and revise their feedback. We also discuss some patterns of PSTs' initial feedback that did change with practice, and other patterns that persisted, suggesting the need for further guidance and practice in providing students with written feedback.