"Dirt will be removed"
U-class nouns (like uchafu) take the u- subject prefix in the singular and zi- in the plural.
If you've seen the aorist tense, which I don't think is used in this course (or if it is, not until late, then yes). The aorist tense is marked with -a- rather than -na- and prefixes before it change a little bit, with u- changing to w-, ki- changing to ch- and i- changing to y- etc.
So there is uchafu waondolewa meaning "dirt is removed (always/in general/inherently)".
Don't suppose you have any online resources for Swahili aorist? Would be nice to have something to point people at for further reading.