"I am a citizen."
Translation:Is saoránach mé.
If you are identifying or classifying something (really think two nouns coming together), you must use the copula is. Forms of bí cannot be used in this situation, without further syntax.
I wrote "Is mé saoránach" and got it wrong. Wouldn't that translate to "A citizen am I"?
No — a classificational copular statement doesn’t use that word order. Saoránach is ea mé could be used to emphasize that I’m a citizen, rather than e.g. a tourist from another country.