Final answer:
The student's sentence is already punctuated correctly and does not require any changes. It properly uses a comma to separate a dependent clause from the main clause.
Step-by-step explanation:
The sentence provided by the student, "Before sending out invitations, Margo checked the party date with her mother-in-law.", is already correct regarding both punctuation and capitalization. The sentence contains a dependent clause "Before sending out invitations," followed by the main clause "Margo checked the party date with her mother-in-law." The comma after the dependent clause correctly separates it from the main clause. Therefore, no changes are necessary to this well-constructed sentence.