Quirk says this:
'Used to' is a marginal modal. Its status is intermediate on the gradient running between auxiliary verbs and main verbs.
Used to denotes a habit or a state that existed in the past, and is therefore semantically not so much a modal auxiliary as an auxiliary of tense and aspect. In formal terms, however, it fits the marginal modal category.
It always takes the to-infinitive and only occurs in the past tense.
Used to occurs both as an operator and with DO-support. In the latter case the spellings 'use to' and 'used to' both occur, reflecting the speakers' uncertainty of the status of this verb: an uncertainty, that is, as to whether it is to be treated as an invariable form, like a modal auxiliary; or as a form with an infinitive,
like a full verb.
In the negative, the operator construction is preferred by many in BrE:
He usen't to smoke.
He used not to smoke.
These are preferred in BrE and AmE:
He didn't use to smoke.
He didn't used to smoke.
The construction did . . . use to is preferred to other constructions in both AmE and BrE. The spelling did. . . used to, however, is often regarded as nonstandard. The interrogative operator construction, for example this:
Used he to smoke?
is rare even in BrE. Tag questions also normally have DO-support.
A perfective form of used to, had used to, is occasionally attested.
There is a tendency for speakers to avoid the problem of negating 'used to' by employing the negative adverb never: I never used to watch television.