-그녀는 그렇게 많이 먹지 않아.

-She doesn't eat that much. ​

-아프지 않아.

-I'm not sick. -> It doesn't hurt. 

=> Your sentence has a correct grammatical structure, but this has the correct usage/ expression.

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-난 사과를 좀 사고 싶어.

- I want to buy some apples.

=> Corrected : The sentence is corrected grammatically. 

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