MOST of us feel comfortable with the form 'used to + verb' when used to mean a past condition or habitual practice, as in 'They used to be very close friends' and 'She used to jog early in the morning.' In the first sentence, of course, 'used to' conveys the idea of an activity or condition in the past that's no longer true; in the second sentence, 'used to' conveys the idea of an old habit that had already stopped. In both cases, we're hardly in any danger of tripping in our grammar because 'used to' is clearly functioning as it should — as an auxiliary verb affirming the sense of a past action or state of affairs that had already ceased.

But using 'used to +verb' in negative and interrogative statements, both of which require the form to take the auxiliary verb 'did,' raises serious questions about its grammatical validity. Indeed, how should the two 'used to +verb' sentences above be rendered in their negative form? For the first, do we say, 'They didn't used to be very close friends' ('used' with the '-d') or 'They didn't use to be very close friends' ('use' without the '-d')? And for the second, do we say, 'She didn't used to jog early in the morning' or 'She didn't use to jog early in the morning'?