Why can't I use the contractions "they're" or "we're" at the end of a sentence?
negation - How to choose a proper contraction "it's not" versus "it isn't"? - English Language Learners Stack Exchange
verbs - Does "you're" also qualify as a valid contraction for "you were"? - English Language & Usage Stack Exchange
etymology - What is "won't" a contraction of? - English Language & Usage Stack Exchange
What is the difference between an abbreviation and a contraction?
What is the difference between a contraction and a portmanteau?
What are contractions (words)?
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For example, "I've never met my parents; I have no idea who they're."
or
Is anyone else hungry? Because we're.
Obviously, those are both terrible sentences, but if they're=they are and we're=we are, shouldn't it work?
‘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.
No, you're, i.e. you are (present tense) is different from you were (past tense).
There isn't a common shortening, but it only saves a letter or two!
Contractions are generally flexible enough to transfer to other bases without much confusion:
They're / We're / You're
They've / We've / You've
They'd / We'd / You'd
You can also stack them if you are feeling edgy (and with mixed success):
You shouldn't've
But other than "'d" there isn't a case for adding extra words that fit the truncated part. Just because "were" matches the syntax for "'re" doesn't mean you can drop it into the contraction:
They're going to live here but now they're not
We're going to the show but they sold out
You're my best friend but now you're my enemy
These sentences just don't parse well due to the same word being used for two different meanings without anything but context to distinguish them. The biggest problem is that the context for each is more or less the same:
You were going to live here / You are going to live here
We were going to the show / We are going to the show
You were my best friend / You were my best friend
The could/would problem is similar:
We'd have eaten pizza
But the advantage here is that either one gets close to the intended meaning. Switching between "were" and "are" is too drastic of a change and too hard to see without clarification.
Wiktionary says:
Abbreviation of wollnot or woll + not, negations of archaic form of will.
The Concise Oxford Dictionary of English Etymology agrees:
XVII. contr. of wonnot, assim. of wol not
As to other forms, Etymonline only mentions wynnot:
first recorded mid-15c. as wynnot, later wonnot (1580s) before the modern form [won't] emerged 1660s.
Won’t actually has a pretty interesting and complex history. Ultimately it does come from a contraction of will and not, but it all happened in a rather roundabout way.
It all started off with the Old English verb willan/wyllan, meaning to will, wish, or want. Even in Old English it was used occasionally to denote a future intent. “Ic wille gan” could mean “I want to go” or “I will go”, depending on context.
Now, the thing about negatives in Old English is that they were often reduced:
na(w)ðer = nahwæðer = ne + hwæðer
neither = not + whethernæfre = ne + æfre
never = not + evernabbað = ne + habbað
haven’t = have + notWe nabbað naðor ne hlaf ne wæter.
We have neither bread nor water.
Not comes from naht via noht. Related to nawiht meaning naught, it originally meant in no way, but came to be used as an emphatic form of ne. Subsequently it became unstressed and supplanted ne altogether. This is an example of Jespersen’s Cycle.
All these things combined led to a new negative form of willan, wynnot. The past forms of willan began with wold-, which is where we get would. Under the influence of these forms and the related verb wol, wynnot became wonnot by the late 1500s.
Finally, the modern form won’t emerged by the 1660s as a result of reducing the final vowel in wonnot. It appears to be the first word so contracted; most of the other -n’t contractions we use today (can’t, couldn’t, shouldn’t, &c.) arose in the 1700s, modelled after won’t. In modern English, cannot is the only uncontracted -not compound that survives.
As for the other contractions such as -’ll and -’ve, their history is just as long, though perhaps slightly less convoluted. But that’s a story for a different question. ;)
Also, remember that spelling in Old English was less standardised than in modern English. There were often several equally valid ways to spell the same word, especially when you took different accents and dialects into account. So sometimes it’s difficult to get a good historical account of pronunciation and usage changes. Still, as far as I can tell, this is basically how it went down.
Source: The Online Etymology Dictionary.