In correcting students' papers, I basically use three abbreviations:
i: idiomatic mistake. Idiomatic English uses words in a way that sounds natural to native speakers.
c: Your sentence construction (=grammar) is not acceptable
as a whole or in detail.
st: The stylistic elements, the words, phrases and/or
structures, you used to express your ideas are not good enough. It means
that you haven't paid a lot of attention to the work we did in class and, as a consequence, to the way you write
about the subject.
