“Wrong”, “Incorrect” or something like it seems good to me.
Also, icons like 
and 
would help.
I still think that using two colors - red for Again and green for other buttons - would be helpful.
For your onboarding goal, I prefer having only two buttons as the default (Pass/Fail Grading as Default - #20 by suiyuan), so someone would only use Hard and Easy if they deliberately enabled it.