10 questions in a row where my answer is rejected because I used articles. So i decide to answer this one without using an article... Ninapenda Kiswahili, but these lessons are frustrating.
I always use Sticky Notes while on a laptop.
"Esther's not the teacher on duty" should be accepted. The given "Esther is not a teacher on duty" sounds unnatural; in English we would rather say "Esther is not on duty" or "Esther is not one of the teachers on duty", if there is more than teacher.