Every time there is a list of three or items in a sentence, those things must be separated by commas, conjunctions, or both. The second and third items of the list must always be separated by a comma, or a conjunction and a comma. The conjunction is missing from between the second and third items in the list above, and this should be corrected.

Here, "finally" is considered an introductory word, so it must be followed by a comma: "Finally, the teacher . . ."

The phrase "If you decide to go to the beach" is an introductory phrase, and all introductory phrases need to be set apart from the rest of the sentence by a comma. So, after being corrected, the sentence would read, "If you decide to go to the beach, make sure to wear sunscreen to protect your skin!"

Because the sentence states that Margot is not Daisy's only sister, the commas and which frame her name in "their numerous older and younger sisters, Margot, and Daisy" are incorrect. The comma that sets apart Margot's name at the beginning of the sentence ("Daisy and her sister, Margot, shared") is correctly used, because here, Margot is being specified as Daisy's sister in an appositive phrase that renames "her sister" as "Margot."

"The treasurer" here is an appositive: it renames or restates Shindra in different words, and it should be surrounded with commas. The correct sentence should be: "the students voted Shindra, the treasurer, president . . ."

Here "despite being an old city" needs to be separated by the main clause by two commas, not just one.

The word "however" here is considered a non-essential phrase: it can be removed, leaving a complete sentence. So it must be surrounded with commas: "The project leader, however, continued . . ."

Commas should be used to set off the phrase "the lost princess," as it is in apposition to (gives a description of) the subject Perdita, but the sentence still makes sense without it. So, "Perdita the lost princess," contains the sentence's error, and the sentence should read, "Perdita, the lost princess, was found and raised by a shepherd."

In this sentence, "First" is used as an introductory word (specifically an adverbial phrase) and must therefore be followed by a comma: "First, take . . ."

The phrase "the medieval outlaw who stole from the rich and gave to the poor" is an appositive phrase. It restates the noun "Robin Hood" in a way that presents more details, but is not grammatically necessary to the sentence for the sentence to be complete and understandable. ("Most schoolchildren have heard the tale of Robin Hood" is a completely correct sentence on its own.) The appositive phrase should therefore be preceded by a comma ("Robin Hood, the medieval outlaw who stole from the rich and gave to the poor").