Here are the comma rules used in each sentence:
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Leo ran down the block, across the street, through the park, and into the library.
- Rule: Use commas to separate items in a series.
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If I win tickets to the concert, I will go with my best friend.
- Rule: Use a comma after an introductory clause.
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We visited many of Rome's landmarks, but didn't get into the Colosseum.
- Rule: Use a comma before a conjunction that links two independent clauses.
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Mom flew to Chicago and landed at O'Hare International Airport, one of the busiest airports in the U. S.
- Rule: Use a comma to separate an independent clause from a non-essential phrase.