Linguistic modality is the expression of the speaker’s subjectivity including possibility, probability, necessity, obligation, permission, prohibition, and desire. This paper analyses a learner English corpus collected at two Spanish universities, paying special attention to which linguistic devices (e.g., modal verbs, adjectives, adverbs or nouns) English as a Foreign Language (EFL) students make use of when providing for and against arguments in their assignments. Applying a corpus-based methodology not only enabled comparisons to be made with other native and non-native data but also facilitated both quantitative and qualitative analyses. The findings show remarkable similarities and differences, and leave several issues at stake: the relationship between the degree of assertiveness of a text and (1) the student’s gender, (2) their command of the Foreign Language (FL), and (3) their familiarity with the genre they are expected to write in.