"I am not a man."
Translation:Mi ne estas viro.
Esperanto uses the copula much as English. The infinitive is esti, and the whole conjugation is regular (as with all Esperanto verbs). In addition, adjectival roots can be turned into stative verbs: La ĉielo bluas. "The sky is blue."
So, it appears you need a verb, but it isn't always going to be estas.
Of course, because it is not the object for this lesson, you should learn the basic use of "estas". But you can convert any noun into a verb.
Unu is used as a number, not as an article. I am not one man = Mi ne estas unu viro.
No because "esti" (and for that matter, the way to say "to be" in every language I know how to say it in: English, French, Spanish, German, Russian, Irish) is different than all other verbs in that it's right-hand side is not a direct object, so you don't put it in the accusative case.
I keep trying to elaborate on this and every time I write out some reasoning, I end up thinking it's actually more confusing, so I keep deleting it. Let me know if you'd like some reasoning and I can try again to write up some analogies!
I've read in some places that Esperanto sometimes allows you to use a noun as a verb to mean "to be (the noun)". Certainly this is true of adjectives (mi bluas = mi estas blua). "Mi ne viras" was marked incorrect, and I'm wondering if I've got that wrong or if I should submit that as a correct answer