by DonGately » Wed Dec 20, 2006 12:47 pm
My posts my not indicate it, because I often type quickly and proofread poorly, but I have a passing interest in grammar (there will probably be a number of mistakes in this post). I looked up "It isn't" vs "It's not" in my handy copy of Garner's Modern American Usage and found... no entry. This either means that it's a matter of personal choice or that Garner hasn't gotten around to writing the rule for this case.
I think it boils down to emphasis. For example, if I say "It isn't important" I mean something different than if I say "It's not important". Both phrases are structurally correct, but, in my case, the former implies that something isn't important right now (but may be important later), while the latter implies that something is insignificant.
Over on Word Reference Forum, most people agree that it's a matter of personal choice. Someone posted the following example and rule based on the idea that the choice is determined by "how...the sentence is going to sound in that final pre-speech mental process":
"Is it raining? No, it isn't.
It's not raining."
The writer then posts "...at the end of a sentence I would say it isn't. But when there's more to come, I would say it's not...."