Sometimes important context or information pertaining needs to be added to the sentence, but the author may not wish to integrate this information into the fundamental grammatical structure of the sentence. In these instance, such information can be contained in what are called interrupting phrases, these are words or sets of words that interrupt a clause, but exist outside of the fundamental structure of that clause. Interrupting phrases must be enclosed with commas. The best version of the example sentence reads, Paulus Atredies, one of the greatest men who ever lived, died fighting a bull in a dirty pit.

Introductory phrases precede and are subordinate to a sentence's main clause. Introductory 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. In the example sentence, "When Franklin treats his mother that way," acts as such an introductory phrase.

Interrupting phrases are subordinate and add information to a sentence, but are not necessary to the sentence being complete. Thus, such phrases must be separated from the rest of the sentence by commas. The best way to correct the underlined portion above is, "Heidi, my college mentor, has often disappointed me after I graduated."

Introductory phrases precede and are subordinate to a sentence's main clause. Introductory 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. In the example sentence, "While I appreciate your commentary," acts as such an introductory phrase. You can often spot introductory phrases because they begin a sentence with a coordinating conjunction, such as "while."

Interrupting phrases add information or context 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 example sentence "one of the kindest women" is performing this function.

Interrupting phrases are not necessary to the sentence being grammatically complete, and they must be separated from the rest of the sentence by commas. Such phrases add information or context, or, as is the case here, rename a noun (these are also sometimes called appositives) The best way to correct the underlined portion above is, "Anna, my ex-girlfriend, is getting married to a guy I don't really like very much, but I guess that's how it goes."

Introductory phrases precede and are subordinate to a sentence's main clause. Introductory 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. In the example sentence "When Suzy is so petulant in class," acts as an introductory clause by providing the condition and context for Suzie's actions. The example sentence is thus missing a comma at the end of this phrase.

Introductory phrases act as dependent clauses, and thus must be separated from the rest of the sentence by commas. In this case the subordinating conjunction "while" tips us to the introductory clause in this sentence. The best way to correct the underlined portion above is, "While I have to admit that she is a great writer, she isn't much of a conversationalist."

In the example sentence the interrupting phrase (or appositive) "Michael" renames the subject of the sentence, the noun "my teacher." Because this interrupting word is not part of the fundamental grammatical structure of the sentence (you could remove it and the sentence would remain grammatically correct) it must be separated with commas on either side, not just one comma at the beginning.

The correct version reads, "My teacher, Michael, is one of the best teachers I have ever had."

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."