What is the meaning of the bare participle "gelernt"? Why doesn't it need to be "hat gelernt"?
The English sentence is the same; if it had been "I have learned nothing again" then you would have said "Ich habe wieder nichts gelernt".
Ah, okay. Duolingo hadn't taught the passive voice in general by this point, so I didn't know that it's formed with the participle.