"We will leave our town tomorrow."
Translation:Tutatoka mjini kwetu kesho.
kwetu is the -etu (possesive "our") that is conjugated with the KU location-subject. Ku+etu = kwetu. The object in this is Mjini, which is mji+ni. The "ni" turns mji from the actual object of a town, into "mjini" the idea of the town as a location/place. For locations you use the Pa, Ku, or Mu. Pa being for specific locations, Ku being for general locations, Mu for being located INSIDE something. Since we're talking about the general location of a town, we use Ku. Ku+etu = kwetu. Mji wetu. Mjini kwetu.
Hope this helped!
-ondoka and -toka almost always take a location phrase rather than an object.
You could say Tutaondoka
katika mji wetu ... or using another word instead of katika (such as kwenye or ndani ya) but I don't think it's really correct to just have mji wetu without turning it into a locational phrase.