If used to is a set idiomatic phrase, (ie not a tense) then why would it change its form from "use to" to "used to" for the sentence as it does in the positive. Ie. Why not say "I use to smoke". "Did you use to smoke?" "I didn't use to smoke".
Chambers's 1939 dictionary tells us that "use to" is an intransitive verb meaning "to be accostomed to" only used in the past tense and pronounced /ust/. Also in Practical English Usage, (Oxford),Michael Swan says that the formal form of the question and negative of "use to" is "Used you to go to the opera?" and "I usedn't to play football" etc.
In Ireland we've have remained faithful to these concepts and often "Quazi-modalize" the question and negative by saying "Usen't you go...?" and "I usedn't play..." etc. The problem I believe is that everybody argues the case in favour of what they themselves are, if you pardon the pun! "used to" saying themselves.
Unfortunately, English does not have an academy of experts that meet once a year, unlike Spanish.
Thanks so much.

If "used to" is a set idiomatic phrase (i.e. not a tense), then why would it change its form from "use to" to "used to" for the sentence as it does in the positive? I.e. why not say "I use to smoke". "Did you use to smoke?" "I didn't use to smoke".

Chambers's 1939 dictionary tells us that "use to" is an intransitive verb meaning "to be accostomed to" only used in the past tense and pronounced /ust/.

Also in Practical English Usage, (Oxford),Michael Swan says that the formal form of the question and negative of "use to" is "Used you to go to the opera?" and "I usedn't to play football" etc.

In Ireland we've have remained faithful to these concepts and often "Quazi-modalize" the question and negative by saying "Usen't you go...?" and "I usedn't play..." etc. The problem I believe is that everybody argues the case in favour of what they themselves are, if you pardon the pun! "used to" saying themselves.

Unfortunately, English does not have an academy of experts that meet once a year, unlike Spanish.