The primary purpose of the paper is to examine how undergraduate writers adopt various commanding strategies of 'shouldness' in their expository essays and the extent to which their adoptions relate to the success in the assessment of essay writing. The theoretical bases of the commands operating both within and across clause complexes are derived from the work of Iedema and Martin and formulated within a Systemic Functional Linguistics (SFL) framework. In the process of applying the theory, some extensions and reworkings of the categories were required. The analysis reveals significant grade-based differences related to the extent to which writers make interactional choices expressing 'command' both within and across clause complexes. These differences are reflected in the use of both the strategies of involvement by mitigating command and the strategies displaying distance by showing formality. The main implication for teaching EAP (English for Academic Purposes) courses is the importance of foregrounding the social interactive nature of an academic argument. Theoretical implications will also be discussed.