A few months ago we discussed how a single comma can change the meaning of a sentence. But commas are more versatile than that; they can also change the meaning of specific words, simply by being placed nearby. For instance, the word while:
We’ll go to the theatre while you watch the baby.
I am a doctor of medicine, while you are simply a quack.
See that? In the first example, while meant “during the time that” and was not preceded by a comma. In the second example, it meant “whereas” and was preceded by a comma.
Now, it’s technically not the comma that changes the meaning of while. But that comma will pretty much always follow this pattern. If you’re using while to mean whereas, then it should be preceded by the comma; if you’re using it to mark a period of time, then there shouldn’t be a comma.
What I’m saying here is that editors don’t want to see this:
He’s throwing a party, while his parents are out of town.
This happens all over the place, and it is incorrect. Resist the impulse to place a comma before a while that’s supposed to indicate a duration of time. Just don’t do it.
The problem with this error is that it still creates a grammatically valid sentence. A normal grammatical error just says something poorly, but at least it’s usually still what the writer meant to say. This error, on the other hand, completely changed the meaning of the sentence from this:
He’s throwing a party during the period of time that his parents are out of town.
He’s throwing a party; his parents, on the other hand, are out of town.
See the problem here? If you’re a writer, someone who makes a living (or wants to make a living) purely by the words you string together, then you need to know how to make those words say exactly what you intend them to. So remember:
Note: When while is being used to indicate a period of time, it is usually possible to switch the order of the sentence’s clauses like this:
While his parents are out of town, he’s throwing a party.
In this case, you will need a comma between the two parts of the sentence, but not beside while.