‘An A-Z of English Grammar and Usage’ by Leech and others suggests the following:
With be, use the contraction + not (e.g. That’s not right).
With have and modal auxiliaries, use the verb + n’t option, e.g. hasn’t, can’t.
The authors go on to say that forms such as She isn’t hungry, as opposed to She’s not hungry, are less common, and that forms such as I’ve not met him, as opposed to I haven’t met him, are much less common.
The authors don’t say on what basis they make their recommendations, but they are likely to be based on frequency of use. There doesn’t seem to be any difference of meaning or emphasis. I would just add myself that a contraction such as I’ve not met him sounds more formal, perhaps because of its lower frequency.
Answer from Barrie England on Stack Exchangenegation - How to choose a proper contraction "it's not" versus "it isn't"? - English Language Learners Stack Exchange
What is the proper contraction of "it has"?
What is the best way to use contractions in the phrase "I have not"?
Is "it's" an acceptable contraction for "it has"?
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‘An A-Z of English Grammar and Usage’ by Leech and others suggests the following:
With be, use the contraction + not (e.g. That’s not right).
With have and modal auxiliaries, use the verb + n’t option, e.g. hasn’t, can’t.
The authors go on to say that forms such as She isn’t hungry, as opposed to She’s not hungry, are less common, and that forms such as I’ve not met him, as opposed to I haven’t met him, are much less common.
The authors don’t say on what basis they make their recommendations, but they are likely to be based on frequency of use. There doesn’t seem to be any difference of meaning or emphasis. I would just add myself that a contraction such as I’ve not met him sounds more formal, perhaps because of its lower frequency.
As you've said, placing emphasis on a word is a matter of desired emphasis which is a matter of personal style rather than rules. I think you could make exactly the same point by putting stress on isn't (It ISN'T my fault!)
In the Python sketch, the emphasised word is 'it' not, 'not'. Part of the humour in the sketch is the repetition of it - 'It is.' 'No, it isn't.' 'Yes it is.'. This wouldn't have carried the same impact if the humour was based on, perhaps, the back and forth of 'It's not', 'Yes, it is.' 'No, it's not.'
Also, they can do this with a number of forms of the sentences in the sketch, for example 'It can be.' 'No, it can't ...' The humour wouldn't carry through these sentences if the emphasis had been used on the 'Is/Not' structure.
My first instinct is that it is "it's", but I was also wondering if it is even proper to make it a contraction at all.
I've been wondering this for a while now and decided to ask you guys what the answer would be. I'm thinking it's either "I've not" or "I haven't", but I have no idea which one is right (or if there is another answer)
Thanks!