How about "Mlango ni chafu"?
that is 'the door is dirty'
Why not "ina" if door is an object?
Mlango is singular, class three so its verb will take the u-prefix (mlango una...). I-na will be used in the plural (milango ina...- doors have).
I thought "lina" was used instead of "una" when translating the other way: Mlango lina uchafu. Should that not be right, too?
the prefix is entirely dependent on noun classes. There's a post on this in the Masterpost