How well can you use proper grammar, and what grammatical tendencies might you exhibit in your writings?
I have a knack for using perfects, which sometimes works against me.
It's obviously impossible for you to determine your own competence for writing with proper grammar, so you can use benchmarks as a means of branding your ability.
A relevant composition on the relationship between an individual's overall linguistic understanding and their grammatical capabilities: http://elib.uni-stuttgart.de/opus/vo.../81/pdf/81.pdf
Bookmarks