Wamesahauliwa means ''they have been forgotten'' too, right? If so, what is the difference between these two?
In reading Joan Russell's The Complete Swahili, it's a matter of emphasis. The -wa form focuses on the fact that somebody has forgotten something (the "they"), the -k- form focuses on the thing being in a state of having been forgotten.
It seems like the difference in English between adjective and participle:
I am dead. I am free.
I am killed. I am freed.
I'm not an expert, though.
^wondering the same thing