Translation:Corinna is not a girl, but a woman.
I think you got things mixed. Elliptical subjects are the ones with omitted or implied parts, and English has plenty of those. For example "John watches more tv than I (watch [tv])" or "Mark plays some instrument, I just don't know which (instrument he plays)" - the versions without the stuff in parenthesis and brackets are elliptical clauses, and not the ones that spell it all out.
Corinna isn't a girl, rather she is a woman should be an accepted translation.
Whereas here, rather is a subordinating conjunction indicating contradiction. It should be encompassed by the meaning of 'sed'
In fact, this is a more specific translation, as in Latin i believe it is only used as a conjunction, and the english word but can be used as an adverb or preposition with different meanings.
There was only one "is" when I did this. Literally, shouldn't it be "Corinna isn't a girl, but she is a woman?"
The expected answer is more elegant, and possibly even more natural. However, when I'm a beginner translating something for the first time, I'm not really trying to figure out how I would best say this in English. I'm trying to understand what is literally being constructed in the Latin and find the matching English words.
There is absolutely nothing "incorrect" about "Corinna is not a girl, she's a woman". It might not be the strictest translation (because part of this lesson is teaching the word "sed"=="but"), and the course contributors might not have included it in the answer database, but there is nothing wrong with it.
The sentence is confusing, because of the lack of meaning (if the woman is a girl, it's weird)
Could you replace your sentence by
"Corinna is not a student, but the woman is"? Or something like that. Your sentence is a bit headache-inductor.
Corinna non est discipula, sed femina.
Corinna non est discipula, sed femina est.
Non Corinna, sed femina discipula est.
Are there other ways to say it?
Scipsit Perce Neige:
1) Corinna non est discipula, sed femina. 2) Corinna non est discipula, sed femina est. 3) Non Corinna, sed femina discipula est.
-- To me, number 1 means "Corinna is not a student but a woman" (and, even though Latin word-order is famously "free", I'd be more comfortable with "Corinna non discipula sed femina est").
-- Number 3 I'd read as meaning "The woman, not Corinna, is a student".
-- While number 2 (with its two "est"s) just doesn't feel right somehow.
But then, quid enim scio? I'm just returning to Latin after having "dropped" it at school 59 years ago -- so you'd be perfectly justified in taking my observations cum grano salis. :)
I should have added to my last comment the observation that there were no commas (or very much else at all in the way of punctuation) in Classical Latin, which does add to the inauthentic feel of some of the sentences Duolingo has invented for its lessons. It's all the more reason why we should be introduced as quickly as possible to the other, textual and sentence-construction methods by which Latin made explicit the subtleties of interpretation which modern European languages convey by means of punctuation.
It's called ellipsis. It's very common to omit the repeated element in the second clause when it's parallel construction with the first clause. But beyond that, different languages have different grammar structures and different ways of saying things. The English and the Latin don't need to be in perfect lockstep with each other.
Word order in Latin is relatively fluid, so translating word for word can end up in a weird or even incorrect translation. So your best bet is to adopt the phrasing that is most natural to the language you are translating in (while if possible keeping the emphasis effects the variations from canonical order in Latin sometimes convey).