Is "Io" mandatory for "I fly"?
I selected "Volo" and "Io volo" in a multiple choice question. I was marked right during the lesson, but "Volo" was marked wrong in the lesson review. Is "Io" mandatory for some reason, or does Duo have a glitch?
Io is never mandatory. It is used when you want to stress that it "I" that flies and not somebody else. It is also often used in the subjunctive because the I, you and he/she and you (singular formal) forms of the verb are the same.
In what form did you have this question? I see it as accepted in the incubator. It does look like a glitch.
It was a multiple choice "select ALL answers that apply." It was marked right when I was doing the lesson, but I noticed it was marked wrong in the lesson review.