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I just never liked that shortcuts in any language, so I try to understand is it a rule or just option in English.
I am a linguistics student and just started thinking about this. I think it might be something syntactical but I could be wrong. It seems in the second expression we would say "You did, did you not?" Is this am issue of raising?
Barren Brainerd published a paper - The contractions of not: A historical note.” Journal of English Linguistics 22:176-196 in 1989, in which he noted that the contraction of not first appeared in writing in 1700s, increased in 1800s, and were more or less accepted in 1900s.
The actual paper which Brainerd published is not accessible, but here is the source which talks about it. Refer to page 26.
The OED’s earliest citation for don’t under the entry for do is dated 1672 and for didn’t, 1775. The earliest citation for won’t under the entry for will is 1710.
‘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.
"Don't", "wouldn't" and "couldn't" are contractions of "do", "would" and "could" with "not".
From the Wikipedia page on Contraction:
An informal type of contraction occurs frequently in speech and writing, in which a syllable is substituted by an apostrophe and/or other mode of elision, e.g., can't for "cannot", won't for "will not". Such contractions are often either negations with not or combinations of pronouns with auxiliary verbs, e.g., I'll for "I will".
- The contractions (e.g. don't) and the full phrases (e.g. do not) have the same meaning.
- Contractions are more frequent in speech than writing.
- Contractions are more frequent in informal than formal contexts.
- It is not always the case that you can replace "don't" or "can't" etc. with "do not" or "cannot" directly; e.g. "Why can’t I?" (See nohat's comments below)
But don't you think that sometimes, "don't" works better than "do not"? "Do not" conveys an urgency or an order. While "don't" is not as pressing or commanding. For instance, I don't see your point of view." It could sound strange to force the argument by saying, "I do not see your point of view." Any thoughts on this?