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." 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"? The strongly recommended usage for negative "used to" statements is therefore this: "They didn't use to be very close friends." For negative "used to" constructions, these grammarians recommend this form instead: "They used not to be very close friends."

Source: Manila Times June 07, 2023 16:23 UTC