Here are your corrected sentences and easy explanations just for a 7-year-old:
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Corrected: I go to space, explore new planets, and work in zero gravity!
Why: When you list three things you do, put a comma before the word "and" so the list is clear.
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Corrected: I fly a rocket into space, explore new planets, and work in zero gravity!
Why: Use "into space" to show you are moving into space. Also add the comma before "and" in the list.
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Corrected: Hi, Earthling! I come from a galaxy far, far away. Let's be friends!
Why: Put a comma after "Hi" when you say hello. Capitalize "Earth" and you can say "Earthling" for someone from Earth. Use commas in "far, far away" for the repeated words and the apostrophe in "Let's" for "let us."
Great sentences — very imaginative! If you want, I can help make them even more exciting.