"The nose smells"
I think this is trying to say that the nose smells [constantly, habitually, regularly, all the time]... hence Pua [noun class 9] hu-nusa, where hu- is for habitual or regular... but "the nose smells" seems like a direct translation should be Pua i-nusa or Pua i-na-nusa... is this not acceptable?
the sentence is in present simple tense, habitual, so the infix '-hu-' is used. The infix '-na-' is used in present simple tense for non-action verbs ('I like bananas' - 'Ninapenda ndizi') or for verbs in present continuous tense i.e. 'the nose is smelling' - pua linanusa
'Pua' is in the noun class 'Ji/Ma', and if you want to exclude the '-na-', you'd say 'pua
lanusa'. For the noun class N/N you'd say '