"Under the teacher's guidance" is an introductory phrase, one that conditions the action of the sentence, but is apart from its main structure. Any introductory phrase needs to be set apart fom the main body of the sentence by a comma. The only answer choice that correctly uses a comma to separate the phrase is "Under the teacher's guidance, the students' reading ability improved."

Introductory or interrupting phrases -phrases that are subordinate and add information to a sentence, but are not necessary to the sentence being complete- must be separated from the rest of the sentence by commas. In the sentence above "Thor the norse god of thunder is" should read "Thor, the norse god of thunder, is."

Introductory are subordinate and add information to a sentence, but are not necessary to the sentence being grammatically complete, must be separated from the rest of the sentence by commas. "Whenever I'm exhausted," establishes the contextual condition for the situation described in the main clause, and acts as an introductory clause. Thus, a comma must be added after "exhausted."

The phrase "a respected team leader" is what is known as an interrupting phrase, or a phrase that provides information but is unnecessary for the structure of the sentence. All interrupting phrases must be set apart from the rest of the sentence by commas; therefore, the correct answer is "The athlete, a respected team leader, was."

The phrase "More than anyone else he had encountered" is an introductory clause, a clause that conditions the information provided in the main body of the sentence. All such clauses need to be set off from the rest of the sentence by a comma, making "More than anyone else he had encountered, the preacher was convinced of the rightness of his cause."

Interrupting phrases are so-called because they interrupt clauses in order to provide information, context (they can also rename a subject, in which case they are sometimes called appositives). In the example sentence, the subject is first named with the proper noun "Larry," then renamed with the label "my favorite comedian." Because this renaming is not essential to the fundamental grammatical structure of the clause in which it appears, it must be separated using commas at the beginning and end of the phrase.

The phrase "Under the old regime" is what is called a conditional phrase, as it gives the conditions which make the main body of the sentence possible. Such phrases always need to be set off from the main part of the sentence by a comma; therefore, "Under the old regime, the action was," is the correct answer choice.

Interrupting phrases add information to a sentence, but are not necessary to the sentence being grammatically complete, and such clauses must be separated from the rest of the sentence with a comma on either end. Generally, such phrases provide context, information, or act as appositives (renaming the subject noun).

The best way to correct the example sentence is, "The young recruit, clearly the top of his class, had great promise."

The phrase "long time veterans of the company" is an interrupting phrase, one that adds information to the sentence but remains outside of its main structure. An interrupting phrase must be set apart from the rest of the sentence by commas. The only answer choice that correctly sets apart the phrase is "The entire staff, long time veterans of the company, was surprised by their CEO's sudden resignation.

Introductory or interrupting phrases, i.e. phrases that are subordinate and add information to a sentence, but are not necessary to the grammatical completeness of the sentence, must be separated from the rest of the sentence by commas. The best way to correct the underlined portion of the sentence above is: "While I can empathize with the plight of your lizard, I find..."