When punctuating a two-part sentence, you should treat the second part as if it were the start of a new sentence. Therefore, if an introductory word group follows a coordinating conjunction or a semicolon in the second part of a two-part sentence, then you should use a comma after that introductory word group.

Question

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In a two-part sentence, if the second part begins with an introductory word group after a coordinating conjunction or semicolon, a comma is used to separate the introductory element from the main clause, similar to how it would be used at the beginning of a new sentence.

When Punctuating a Two-Part Sentence, You Should Treat the Second