"I left after we ate."
Translation:Je suis partie après que nous avons mangé.
You mean why is it "partie" instead of "parti"? It agrees in gender, so in this sentence if the speaker is female, it will be "partie", and if the speaker is male, it will be "parti". Both are accepted in translation from English to French, of course.
Thanks. I must have made another error then, because I entered "parti" and was marked incorrect, even though the "parti" was underlined. But, not a problem, because you've confirmed my understanding.
That's not unlikely, Duo's algorithm is notorious for underlining the wrong word!
"nous ayons mangé" is the subjunctive past, "après que" is followed by indicative.