This study examines the functional variability of so in essays written by 200 L1 English speakers (ENSs) and 400 Japanese EFL learners (EFLs). Using quantitative and qualitative approaches, this study elucidates discourse marker usage of so in each group, thereby establishing the normative patterns of use among ENSs and the features specific to L2 English writers. The findings suggest that ENSs use so strategically as a preface to stance-taking by carefully selecting and adjusting the information to be established as common ground with the reader. EFLs use so in a manner distinctly different from ENSs, displaying varying degrees of understanding and difficulty in utilising the word’s discoursal properties. The study concludes that it is important for L2 English learners to learn the uses of so not only as a connective marker with resultative meaning but also as a resource for projecting stance and assertion.