From Bartleby

"We use the verb use in its past tense with an infinitive to indicate a past condition or habitual practice: We used to live in that house. Because the -d in used is not pronounced in these constructions, people sometimes mistakenly leave it out when writing. Thus it is incorrect to write We use to play tennis. When do occurs with this form of use in negative statements and in questions, the situation is reversed, and use to (not used to) is correct: You did not use to play on that team. Didn’t she use to work for your company?"