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)".

U-class-nouns in the singular have the concordance of class 3 (m- such as mti) and in the plural the concordance of class 10 (zi-, nyumba)