Is there really any difference between "it's not" and "it isn't"? They both mean "it is not", right? Why would you use one or the other?

Just wondering... .

To me "it's not" is more awkward, and "it isn't" is less so. That's it. There are probably some grammar experts around here that know, but when I am teaching, I usually tell students to try and see which one sounds best. I teach the other rubbish as well, but that is as a start.

I use 'it isn't' because it sounds more 'natural'. Sometimes if you are not sure it can help to use the 'just feels right' in your stomach. If you wince when you hear it, there is a good chance it is probably wrong.

At least that is what I do when I am not sure.

I've never really thought about this - I don't know if there is a specific rule. It's not that there COULDN'T be one, but it isn't too clear to me what it would be.

I have a rule about which one sounds better but it came about from an office joke. This guy we didn't care for much had a pronounced accent in English and he was being inspirational and said, 'I don't want to hear anyone saying, It's not my job.' But, the way he said it with a Steve Martinesque French accent sounded like snot.

So, a couple of clever folks made little badges saying, 'It snot my job'
there's my reason!

It would be interesting if someone could provide a definite rule on these two examples. Personally, I prefer "it isn't", as "it's not" always puts me in mind of a slightly crude pun I once heard, which goes "I thought my nose was bleeding, but it's not"!

I've discussed this with a friend of mine who just recently ended his tenure as our state's Poet Laureate. I used to wonder about it while I was working on my last book, and his opinion, which I really value, was that "it isn't" is the way most people speak the phrase. "It's not" is more often used in writing it. That works for me. Speaking, I tend to instinctively say "It isn't," while writing, my fingers just naturally type "It's not."

This brought to mind a similar difficulty experienced by many learning English: the phrase "I am not" is correctly shortened to "I'm not" but saying "I amn't" is definitely incorrect. Why are both versions of "It is not" correct, but not both versions of "I am not"?

I have practiced saying them aloud and think that I probably say "it's not" but might write "it isn't", actually thinking about it, I probably say both. When I speak it doesn't run together in that unfortunate manner, it is definitely two words.

The Concise Oxford Companion to the English Language mentions both it isn’t and it’s not, but doesn’t indicate any difference except to say that it’s not is “more common” (I’m not convinced that this is true).

I think I probably use both forms indifferently, in speech and writing, and I can’t come up with any reason why I might prefer one to the other.

Sometimes I may be influenced by balance or symmetry. In the past tense, there’s no alternative to it wasn’t: so I might say of a defective machine “It wasn’t working yesterday and it isn’t working today.” But if I’m just referring to the present, I think I’m more likely to say “It’s not working.”

Well, I guess the concensus (from here and discussing it with friends)is that it seems it's mostly a matter of personal preference. I've been listening to myself for the past couple days, and can't see any pattern to it, really. Except perhaps for TabbyTom's time dependent example. It seems I use them that way too. Hmm...

That eases my mind, as I have an annoying number of rules of grammar stuffed in my brain and it bothered me that I couldn't come up with one for this.

(Notice I didn't say I necessarily follow those rules. I tend to write (in forums and letters, not formally) the way I would speak--which is rather casually. )

I tend to skip "it's not" and/or "it isn't" and just go with the usually applicaple "it ain't". But, then, we talk funny in Ohio ...