How to construct negative "used to” sentences

In my column last week on how the English double possessive works, I clarified this baffling usage that linguists call the "double genitive." This time I'd like to make a similar alert on another tricky English usage — the construction of the "used to + verb" form in negative and interrogative statements.

As we know, the "used to + verb" form denotes a past condition or habitual practice, as in "She used to be my trusted associate" and "The couple used to swim in the community pool." In the first, "used to" conveys the idea of a condition that's no longer true; in the second, it conveys the idea of an old practice that's no longer being done.