I looked up the following sentences in M. Swan, I quote:
[i]Did you use to play football at school?
I didn’t use to like opera, but now I do.
These forms are not often written, but when they are, they are sometimes spelt did…used to and didn’t used to; many people consider these spellings incorrect.
He also says that in a formal style, the following sentences can be used:
[b]I used not to like opera but now I do.
Used you to play football at school?[/b]
Swan says that these sentences are very rare.[/i] What do you think? Also is ‘Didn’t you used to play football at school’ a correct question?
I think I’d rather use ‘did…used to’ and ‘didn’t used to.’
This is an area of uncertainty in the language. I think these constructions are best avoided altogether in writing, if at all possible. For me, “Did you used to…” looks intelligible but somewhat illiterate, while “Did you use to…” just looks meaningless because I don’t perceive “used to” (in this sense) as an inflection of “use to”. Opinions vary; my opinion may be a minority one.
I’m surprised at “very rare” for the first one. To me (BrE), that’s fine, and I would naturally use it in conversation. The second is technically correct English but feels stilted.
I would used the first two but not the last two, which feel pretty strained (even if technically correct).
You can’t use “used you?”, “used I?” or similar as tag questions. You can say “You used to …, didn’t you?”, “I used to …, didn’t I?”, etc… If the main sentence is already a question (like your “Used you not to go there when you were young?”) then you can’t also use a tag question at all.
To me, “usedn’t” looks fairly ugly written down, even though you can no doubt find plenty of instances of its use.