1. 아주 좋아.

It's very good.  ------>​  That sounds great.

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

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2. 안녕,덕보야. 어제 뭐했니?

Hi, Dugbo.What did you do yesterday?  ------>  Hi, Dugbo. What did you do yesterday?

(Spacing)

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3. 엄마를 도와 드렸어.

I helped my mom. ✔​ ​​​​​​​​​​​​​​​​​​​​​​​​​​​