English has two idiomatic constructions, both spelled used to,
and also both pronounced /'yustu/ or /'yustə/ -- never /'yuzdtu/.
One is the transitive predicate adjective be used to, always with an auxiliary be,
which means 'be accustomed to', and which can take a gerund complement clause.
- I'm used to smoking three packs a day.
The other one, which is relevant here, is the past auxiliary construction
Vinf, which takes an infinitive complement.
- I used to smoke three packs a day.
This construction asserts that something was true in the past, and presupposes that it is
not true in the present. That's why the two sentences above are close to opposite in meaning.
There are no problems with either idiom in real (i.e, spoken) language;
it is only in spelling that there is a problem, and it is an insoluble one.
In both cases, used to has been reified -- joined together into a single word, like to and gather, or may and be, are joined into the single words together and maybe. This happens all the time; it's how idioms are formed. In language.
But spelling is different. Most if not all English readers have learned that
used is how the past tense and past participle forms of the verb use are spelled
- In a question without an auxiliary verb, one inverts the subject and auxiliary verb
- If one needs an auxiliary verb to invert and there is none, one inserts (and inverts) do
(this is called Do-Support)
- do is an auxiliary verb that takes an infinitive complement.
use is how the infinitive form of the verb use is spelled
But used to is the fixed spelling for the /'yustu/ pronunciation, in both idioms
(rather than /yuzd tu/, as in Shovels were used to dig this entrance tunnel).
So if it isn't spelled used to, it won't be pronounced or recognized right.
But, yet again, auxiliary do requires an infinitive complement, and used just can't be one.
The result is that while
is perfectly grammatical and ordinary spoken English, neither one of these ways to spell it is correct,
- *He didn't used to do that.
- *He didn't use to do that.
in the sense that a fluent reader of English is likely to trip over both of them. They just don't look right. This is a problem in the spelling system only, not the language. The language has no problem at all.
The first looks bad because used looks like a misspelled infinitive,
and the second because use is pronounced /yuz/, not /yus/.
The workaround is to avoid using used to in sentences with Do-Support -- but only in writing.