¿Qué demonios es lo que he hecho mal?
What the heck have I done wrong? Never heard hecho mal as done wrong. Is this right? Gracias.
2 Answers
It is perfect.
Your exact phrase would be, "What the hell is it that I have done wrong?" But Lazarus is right; your idea is correct.
A more common way would be simply "¿Qué demonios he hecho mal?"