I have just learned from what I consider a reliable source, that the following sentence is correct:
One of us is wrong, aren't we?
I would never in my life have written this, but I am assured that this is exactly how it would be written
As I realize comments don't live forever I will quote the relevant parts:
oerkelens : So you would really write One of us is wrong, aren't we? I guess by analogy you would not bat an eyelid at One of these balls is blue, aren't they?
Matt Эллен : yes, "Then one of us is wrong, aren't we?" is exactly how it would be written. Same for the balls.
Could someone please enlighten me how it is possible that the number in a question tag supposedly has to be in discordance with the subject of the main clause?
I admit that I am not the youngest any more, and my school days are long gone, but back in the days, I was taught that 1. a verb and its subject concord in number 2. a question tag concords with the subject of the main clause
I was given examples like:
It is warm today, isn't it?
We will be on time, won't we?
Mary is pretty, isn't she?
John isn't the brightest, is he?
Some people may notice that the subject of the main clause seems to determine every time the subject of the question tag. When the main clause subject is singular, so are verb and subject in the question tag.
I was under the impression that a) this made sense and b) this would be a general rule. I fully realize that grammar does not have to make sense, so a) is immaterial. As for b), today I learned I was wrong :)
So when (and possibly why?) do we form question tags that are different in number from their main clause?
To exemplify, also the other example sentence that I used and was corrected on:
One of these balls is blue, isn't it? (so this is wrong)
One of these balls is blue, aren't they? (and this is correct)
As an afterthought, does this strange grammatical number mix-up only appear in question-tags, or should I always refer to singular subjects in the plural if certain conditions are met? And what are those conditions?
Which versions are correct?
One of the cars is broken, aren't they? They (the car(s?) that is(are?) broken) should be fixed.
One of the cars is broken, aren't they? It (the car that is broken) should be fixed.
One of us must be wrong, mustn't we? We (the one(s?) that is (are?) wrong) should make amends.
One of us must be wrong, mustn't we? He (the one that is wrong) should make amends.
(In the last one, they could of course be used as the singular they, but that would avoid the issue...)