If you think your sentence should be accepted but isn't, use the report function.
In the mobile app (at least the Android version) you can write anything in a report. The web version doesn't allow this.
Ironic that Duolingo, which promotes itself as a greater language teacher, has such a shaky grasp of English. Maybe he will come. It may be that he will come
Maybe Duolingo didn't write the Swahili course itself. The course developers did that, and I get the impression they are Swahili speakers with a less-than-perfect grasp of English. They are volunteers and we are getting it for free. This course is still in beta (in all but name) and progress is slow.
There are native English speakers on the course who have offered to correct the English themselves, but they don't seem to have received any response. Does anyone know if Duolingo does actually allow this? (Maybe there are access issues?)