As far as I know Swahili phonology includes both the prenasalised plosive AND the separate. In this case I am pretty sure that it is as I wrote. The prenasalised plosive tends to occur at the beginning of a word.
Where do you get your information on that from? :)
I put "the students write and it's wrong" it says it should be "the students are writing" I am surprised because previously there was no difference between present simple and present continuous