What is the purpose of "Nos" here?
It's just the pronoun "we", but it is not necessary as sumus tells us that the subject is "we".
Then under no circumstances do as the Romans do.
what would be latin for "We are not roman" ?
(Nos) Romani non sumus would work as a translation.
It accepted the answer "Nos Romae noS sumus"
Are you sure they didn't mark "There's a typo in your answer"?
I don't think we're in Kansas any more, Toto...