The part-whole relationship is sometimes overlooked in fractions learning due to its perceived limitations. However, prior studies have shown that students often struggle to master the part-whole subconstruct and encounter difficulties. Despite these challenges, the part-whole subconstruct plays a critical role in supporting students' subsequent fractions learning. This study aimed to explore how students understand and reason about the part-whole subconstruct across its key components. Using a design research approach, a teaching experiment was conducted with 38 fourth-grade students. Students' written works, transcripts from the teaching experiment recordings, and observational notes were retrospectively analyzed to examine the learning trajectory. Several key findings emerged from the teaching experiment, each corresponding to a component of the part-whole subconstruct. We highlight that (1) learning the partwhole subconstruct involves progressively building upon students' prior fractions concepts across its key components and (2) working with incomplete situations helped students refine their understanding of what constitutes a valid fractional representation. These findings offer valuable insights for improving instruction on the part-whole subconstruct and addressing challenges associated with it. The study concludes with recommendations for future research based on the findings.