-괜찮단다. 내 것을 써라.

Thay's okay.You can use mine. --> That's okay. Use mine. ✔​

=> Another way: Your sentence is okay and this is another way of saying it.

-죄송해요. 책을 안 가져왔어요.

I'm sorry.I didn't bring book. --> I'm sorry. I forgot my book. ✔

=> Another way: Your sentence is okay and this is another way of saying it.

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-수학은 지겨워.

Math is bored. --> I don't have my Math book.

=> Corrected : The sentence is corrected grammatically.​