(temporal location)Formerly and habitually or repeatedly, but possibly no longer, did.
I used to be undecided, but now I’m not so sure. I used to like that band and I still do.I used to know a guy from the UK who pronounced "mother" without the "r".It used to be me, sitting in that chair.There used to be open fields here. Now it's a shopping mall.
They burned the old gun that used to stand in the dark corner up in the garret, close to the stuffed fox that always grinned so fiercely. Perhaps the reason why he seemed in such a ghastly rage was that he did not come by his death fairly. Otherwise his pelt would not have been so perfect. And why else was he put away up there out of sight?—and so magnificent a brush as he had too.
He used to drop into my chambers once in a while to smoke, and was first-rate company. When I gave a dinner there was generally a cover laid for him. I liked the man for his own sake, and even had he promised to turn out a celebrity it would have had no weight with me.
With did as an auxiliary verb (as in the negative and interrogative), use to is considered standard, especially in American English. In other cases, such as I use to go to the fair every year, it is considered an error, motivated by the near (or exact) homophony of use to and used to.
The negative may be formed as used to not or used not to (usedn't to, usen't to), did not use to (didn't use to), or did not used to, the last of which is sometimes considered an error.
The interrogative is constructed like did [subject] use to...? (did [subject] used to is also found, but is considered an error) or used [subject] to...?, varying by region and era.
When it is not necessary to include the following verb, in some regions it is usual to use the verb do as a stand-in (he works harder than he used to do), whereas in others it is usual to use no verb at all (he works harder than he used to).
↑ 2.02.1Stewart Clark, Graham Pointon, The Routledge Student Guide to English Usage: A guide to academic writing for students (2016, Routledge, →ISBN), page 296: "In questions, use Did he use to go to Cardiff? rather than Did he used to go to Cardiff? This second version is clearly non-standard. The same applies to negatives: He did not use to play football is recommended usage, but He did not used to play football is non-standard. Note that the alternative He used not to play football on Sundays is correct usage, but too formal for most contexts."