To explore the affordances of learning progressions (LP) in support of teachers' formal and informal formative assessment (FFA and IFA) practices, we conducted a case study with a fine-grain-sized energy LP. The study first theorises an LP-based formative assessment model, which proposes the use of LPs in clarifying learning objectives, eliciting and interpreting students' understanding, and acting in instruction. Then, we examine multiple iterations of two high-school teachers' lesson plans and their enactment of the lessons to identify the instructional adjustments teachers made as part of the LP-based FFA and IFA after professional training. We found that both teachers refined the learning objectives and activities and most of their adjustments promoted students' learning progression; their adjustments have an interdependency between objectives and activities. Both made more adjustments in the IFA than in the FFA, but the alignment with the LP was higher in FFA than in IFA. Contrary to the researchers' expectations, both teachers perceived the LP as a content structure. However, in practice, both employed it as a reference to interpret students' responses by comparing these responses against their own expectations. The teachers also reported collecting assessment information during teacher-student interactive activities and using this material to infer the level of students' understanding in order to decide on the next instructional activities. Both teachers reported that the process of directly engaging with the LP formatively resulted in their having a much more nuanced sense of students' understanding when they revisited and altered the sequence of the learning activities.