The negative of used to is most commonly didn’t use(d) to. Sometimes we write it with a final -d, sometimes not. Both forms are common, but many people consider the form with the final -d to be incorrect, and you should not use it in exams:It didn’t use to be so crowded in the shops as it is nowadays.I didn’t used to like broccoli when I was younger, but I love it now. (Don’t use this form in exams.)