"His t-shirt is dirty"
Translation:Fulana yake ni chafu
The adjective prefixes for the N/N class nouns are a bit tricky!
Adjectives starting with b, d, j, g, v and z use the n- prefix (nzuri, ngumu etc.)
Adjectives starting with a vowel take the prefix 'ny-: nyusi, nyingi, etc.
Adjectives starting with b, v or p take the prefix m- (mbaya). But p only if the adjective consists of one syllable. So njia mpya, but njia pana.
Adjectives beginning with other letters take no prefix: hence chafu, tamu
And to add to the fun of learning: there seem to be a lot of exceptions to these rules :) (-wili -> mbili, -refu -> ndefu)