"A chair will be sat on by a teacher"
Translation:Kiti kitakaliwa na mwalimu
Probably because the passive isn't really that much used except for general statements, like this very sentence, and for specific modes of writing. Especially scientific writing and reporting, like in newspapers. I don't have much problem knowing what the passive does, but I thought this example was especially strange. Many languages use the passive much more than English does, and in a broader set of circumstances, and when you then translate to English, the translation would necessarily become strange. Of course you should translate more or less directly when you do it to learn another language, but you can't help but be amused by the awkwardness of some sentences.
Verbs ending in a double vowel (like kukaa) take the affix -liw- or -lew-in the passive voice. Just another rule :)
-chagua -> -chugu
-fungua -> -fungul
-toa -> -to
-oa -> -o
I have no idea why this is so, but I hope that this answers your question anyway!