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2.2.1.3.3 used to
It is a helping verb to express habitual or accustomed actions, states, etc., taking place in the past but not continuing into the present. The verb after used to is always in its original form. Here, confusion arises.
Since the verb after used to is always in its original form, is the verb that follows together with the to before it a to-infinitive? If it is a to-infinitive, then the phrase used to should be separated. If used to is a phrase, the to generally is a preposition, and what follows a preposition should be either a noun, a gerund or a noun phrase, like:
“I am opposed to wars in general,” (wars = noun)
“in addition to developing digital photos,” (developing = gerund)
“accustomed to the tune,” (the tune = noun phrase)
Apparently, used to fits into neither case. After some research, I've found that used to can only be taken as an exception to the rules. In prescriptive grammar, as always, there is no lack of exceptions. Similar exceptions include phrases like “in order to,” “so as to,” “in a position to,” etc. For details, see 5.4 “To do or to doing?”
Some grammarians consider used to a verbal phrase or expression; others call this “used” a verb without present tense. We just follow tradition and put it as a semi-modal verb here.
The negative form is either didn't used to or didn't use to; the Collins English Dictionary says in formal contexts used not to is preferred. The question form is did (someone / something) use to. For example:
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I used to come here a lot but not anymore.
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He used to eat meat, but now he's a vegetarian.
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Lily didn’t use to eat sweet potatoes when she was younger, but she loves them now.
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Her mom used not to fuss like this.
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It never used to bother me. It certainly does now.
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Did this building use to be a hotel?