Certain moments in life are worth waiting for: a light breath of spring on the late winter air, the first sip of a treasured vintage, the first plunge back into the flow of a sentence after a noun or pronoun that describes a preceding noun or pronoun ...
What? You prefer to cannonball forward without a pause? Are you trying to give someone a heart attack?
Clarabell Hayes, a trained nurse was on the scene of an accident involving a writer who didn't know when to stop.
Uh-oh. Sadly, the writer was unable to receive help because his readers thought he was telling Ms. Hayes that a trained nurse was present rather than describing her as one.
What's needed here is a comma that sets off the appositive, which is the descriptive phrase:
Clarabell Hayes, a trained nurse, was on the scene of an accident ...
Now Ms. Hayes is properly placed and can respond in her proper capacity.
The reason we need to set off the description (a trained nurse) with commas on either side is that we are simply defining the subject here, stating what it is.
We could take either Clarabell Hayes or a trained nurse and use it on its own in the same place in the sentence. Each can serve the same grammatical role as the other.
Usually, when punctuation gets left off of an appositive, the comma that is mistakenly omitted is the second one, which tells the reader that the definition is over and it's time to get on with what the sentence was going to say. This happens often with work titles:
Bo Deadlock, president and chairman of the board of Dippity Dull Batteries stepped down today under suspicious circumstances.
The foundation offered its highest award to Vi O'Grady, founder of Barkers for Parkers for her service to homebound teens.
If you don't offer that comma as a breather after the word Batteries in the first example or Parkers in the second, the reader has to figure out on her own where the description ends, and that momentary confusion interferes with communication. It is far better to set the reader's mind at ease with punctuation that clarifies where the description begins and ends.
Bo Deadlock, president and chairman of the board of Dippity Dull Batteries, stepped down today under suspicious circumstances.
More rarely, a writer makes no space before the appositive phrase:
The district superintendent a rangy man with a brown beard had never actually visited the school.
This type of omission is still more confusing, because the reader has not been alerted that a definition was coming in the first place. It's hard, at the outset, to decide what's going on, and the reader might easily be misled into thinking that there are two subjects in this sentence, the superintendent and the rangy man, with a missing "and" between them.
The commas here make it clear that the superintendent is solely responsible for the lack of attention to the school under his jurisdiction:
The district superintendent, a rangy man with a brown beard, had never actually visited the school.
Plunge into the murky depths of action and description if you dare, but if you wish to be easily understood, take care to pause for appositives.