The comma in the sentence is incorrectly deployed, as it divides a continuous phrase into two parts. The sentence should actually have no commas in it, as the sentence works as a continual phrase. Therefore, the correct answer choice is "The hardest thing for most new pianists to lean is the correct hand position."

The phrase in quotation marks, "What goes around comes around," is the subject of the sentence, but described as an idiom. This means that it needs to be treated as a full sentence, because it is being referred to as a singular whole. To be a complete sentence, a comma needs to be put between its two parts, making ""What goes around, comes around" is an idiom," the correct answer choice.

The use of the comma in the middle of this sentence is completely unnecessary and weirdly divides the sentence into two. No punctuation is needed in this sentence, and the best choice is to remove the comma. Therefore, the correct answer choice is "Making a great deal of money is rarely a path to happiness."

The sentence does not need more commas inserted into it, but fewer. The sole usage in the sentence is a comma splice, or a random insertion of a comma where it should not be. The correct answer choice is "Many former army commanders came out against the plan in newspapers as soon as the story was leaked."

The use of the comma after "circumstances" is incorrect, as the verb "would" connects the conditional phrase to the rest of the sentence. This means no punctuation is necessary, so "Under no circumstances would he let" is the correct answer.

The phrase "about his new job" is vital to the construction of the sentence, as it provides information about the second part of the sentence; therefore, it should not be set apart by commas. The correct answer is "The worst part about his new job."

In this sentence, “Seth” and “which can distinguish between hundreds of different flower scents” are relative clauses. In other words, they’re non-essential information that can be removed and still leave a complete sentence. Therefore, they must be set off from the rest of the sentence with commas.

Here, “who was born in Ulm, Germany,” is a relative clause that modifies or describes “Einstein,” and “which is important to modern physics” is a relative clause that modifies “his theory of relativity.” Because relative clauses contain non-essential information (they can be removed and still leave a complete sentence), they must always be separated from the rest of the sentence by commas. Also, don’t forget that commas are always used between the name of a city and the name of its state or country, as in “Ulm, Germany.”

Here, “after buying the puppy” is an interrupting phrase and can be removed from the sentence without making a sentence fragment. For this reason, it has to be set apart by commas. There is no comma between “techniques” and “that” because the words “that the book recommended” are necessary to specify which training techniques the sentence is talking about. If the words were removed, readers wouldn’t know which training techniques the speaker's dad followed. So, as with all essential sentence information, this phrase is not set apart by commas. (An easy trick is to remember that commas never appear immediately before or after the word “that.”)

Since “the twins spent all day trying on shoes” and “by the end of the day, they concluded their favorite store was the new boutique that their friends had recommended” are independent clauses (i.e., complete sentences on their own), a comma must be used before the conjunction (“and”) that joins these two independent clauses. “By the end of the day” is an introductory phrase for the second clause, so it must be separated from that clause with a comma.

There is no comma between “boutique” and “that” because the words “that their friends had recommended” are necessary to specify which boutique the sentence is talking about. So, as with all necessary sentence information, it’s not set off by commas. (Remember, commas never appear immediately before or after the word “that.”)