CORRECTED COMPOSITION:

I am Daniel. I live in Ilsan ,Korea. And I'm in the 3rd grade. I have one sibling (one younger sister). My younger sister is sometimes annoying but she gives me the laughs. I have my 2 parents, mom and dad. I think our family are the happiest supposition in the world.

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Daniel: I live in Ilsan ,Korea. And I'm in 3rd grade.
Correct: I live in Ilsan, Korea and I'm in 3rd grade.

EXPLANATION:

-Comma (,) is placed after the word immediately and then space.
-You can just continue the sentence and not put period first.
-You can't start your sentence with a conjuction.

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Daniel: My younger sister is sometimes annoying but she gives me laughs.
Correct: My younger sister is annoying sometimes but she makes me laugh.

EXPLANATION:

-The adjective comes first before the transition word.
-"gives" is to present something voluntarily, so instead of saying gives you can use the word "makes"  because she is the cause of your laughter.

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Daniel: I think our family are the happiest supposition in the world.
Correct: I believe our family is the happiest family in the world.

EXPLANATION:

-Family is considered as one because it is collective noun.
-Supposition means the belief that something is true, even though there is no proof, and that is the same as the phrase "I think" that you used in the beginning of the sentence.

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You did a great job! Just a few things to remember so that next time you will be better! I know you are very smart and you will very good in not time, Daniel! ~

Love and hugs,
Teacher Kathline ^^