5 Sentences Saved By Em Dashes

Sentential adverbs (words such as indeed or namely and phrases like "that is" and "of course"), and their close cousins the conjunctive adverbs, or adverbial conjunctions (however, "on the other hand," and the like), indicate an interruption of thought, and should themselves appear as interruptions. Because they are parenthetical remarks (the framing sentence would be complete without them), they should be set off by commas:

"You must, after all, admit that it was a good effort."

If they are employed to indicate a new thought, stronger punctuation is called for:

"They are highly skilled; however, they do not possess the level of knowledge you do."

(In each case, the adverb could also appear at the end of the sentence — after a comma.)

Often, though, the interruption in sentence structure is somewhere between comma country and semicolon stature: The phrase that begins with the adverb is something more than a dependent clause but not quite an independent clause. In these cases, the linking function of an em dash is appropriate:

1. "I thank them for putting up with this project with such good sportsmanship, indeed with such exuberance."
The phrase beginning with indeed is tacked on to the basic sentence to provide an additional, loosely related thought. Note the shift with an em dash, and follow the adverb with a comma to mark elision of a repetition of the phrase "for putting up with": "I thank them for putting up with this project with such good sportsmanship — indeed, with such exuberance."

2. "There is a job to be done, namely educating educators how to effectively teach that wildlife conservation addresses quality of life for everyone."
The phrase that follows "There is a job to be done" is an explanation of what is meant by that phrase. The traditional marker for explanation is a colon, but an em dash does just as well. Again, set the adverb off with a comma: "There is a job to be done — namely, educating educators how to effectively teach that wildlife conservation addresses quality of life for everyone." (Without the comma, the sentence seems to refer to "namely educating educators," but how do you do something in a namely manner?)

3. "They may also be judicially voided for being unreasonable, that is, unsupported by the evidence claimed to justify them."
A colon is often employed to set off a sentence from a subsequent clarification, but the adverb — and the fact that the clarification is an incomplete sentence — justifies use of an em dash here: "They may also be judicially voided for being unreasonable — that is, unsupported by the evidence claimed to justify them."

4. "Furthermore, a scientific conclusion is based on the past, i.e. previous studies that lead to present conclusions."
The initials i.e. (an abbreviation for id est, Latin for "that is") gives you a clue that this sentence can be treated identically to the previous example. Note, however, that just as you follow "that is" with a comma, set i.e. (and the similar e.g., which means "for example") off from the following phrase: "Furthermore, a scientific conclusion is based on the past — i.e., previous studies that lead to present conclusions."

5. "Ethics, on the other hand, is future oriented, that is to say a present choice is based on a future desire, intent, or consequence."
This sentence contains two adverbial phrases: "on the other hand," and "that is to say." The first one, a simple parenthetical phrase, need not concern us, but the latter is an expanded version of "that is" and needs the same treatment as the short form: "Ethics, on the other hand, is future oriented — that is to say, a present choice is based on a future desire, intent, or consequence."

Thanks to Mark Nichol / Daily Writing Tips
http://www.dailywritingtips.com/5-sentences-saved-by-em-dashes/