As Nina said, it's about the noun class. Here are a bunch of examples.
We already know these ones for the different grammatical persons in the M-WA class.
Nina uchafu. = I am dirty.
Una uchafu. = You are dirty.
Ana uchafu. = S/he is dirty.
Tuna uchafu. = We are dirty.
Mna uchafu. = Y'all are dirty.
Wana uchafu. = They (animate) are dirty.
Mpenzi wangu ana uchafu. = My intimate partner is dirty.
Wapenzi wangu wana uchafu. = My intimate partners are dirty.
Mti wangu una uchafu. = My tree is dirty.
Miti yangu ina uchafu. = My trees are dirty.
Dirisha langu lina uchafu. = My window is dirty.
Madirisha yangu yana uchafu. = My windows are dirty.
Chumba changu kina uchafu. = My room is dirty.
Vyumba vyangu vina uchafu. = My rooms are dirty.
Nyumba yangu ina uchafu. = My house is dirty.
Nyumba zangu zina uchafu. = My houses are dirty.
U class with N class plural:
Uso wangu una uchafu. = My face is dirty.
Nyuso zangu zina uchafu. = My faces are dirty.
Animate nouns (people and animals) use a- and wa- even if they're not in the M-WA class ... with a weird exception for the possessive forms of the N class, which stay as the N class (probably because otherwise they wouldn't indicate plural or singular because they'd both use "w" in singular and plural and since it's not marked on the noun.
Simba yangu ana uchafu. = My lion is dirty.
Simba zangu wana uchafu. My lions are dirty.
Rafiki yangu ana uchafu. = My friend is dirty.
Rafiki zangu wana uchafu. My friends are dirty.
The literal translation of "Dirisha lina uchafu" is "The window has dirt" but in English it is more common to "The window is dirty"
So while English naturally uses an adjective "dirty" , the Swahili uses the noun "dirt" and so the sentence structure differs.
Other examples; "Nina njaa" literally means "I have hunger" but it is more natural to say "I am hungry" . French also literally says "I have hunger" i.e. "J'ai faim " but essentially translated to "I am hungry " in English. I hope that helps.