1. 무슨 일이니?
What's wrong.
=> What's wrong?
2. 수학책을 안 가져왔어.
I didn't bring a math book.
=> I don't have my Math book.
3. 괜찮단다. 내 것을 써라.
It's okay, Use mine. ✔
• Corrected: The sentences are corrected grammatically.