This study applied a Systemic Functional Linguistics (SFL) model to explore how 27 first-year university students in two different English proficiency groups improved their lexicogrammatical choices and metafunctions for writing analytical exposition essays during a 15-week course. To explore how "the teaching learning cycle" influences students' understanding of the target genre essay, a survey was conducted; furthermore, to explore changes in students' understanding of metafunctions (ideational, experiential, and textual meanings) of the target genre essay, students' pre- and post-essays were scored by raters using the SFL framework rubric. Then, six students with lower rating scores at the pre-essay stage from both English proficiency groups were selected to explore how they progressed differently in the target linguistic resources. The results demonstrated that applying an SFL framework of writing assessment to English students' understanding of essay writing can be used to explicitly examine their improvements.