The difference between "used to" and "be used to" is in their usage and meaning. "Used to" is used to talk about past habits, states, or facts that were true before but are not true now. For example, "I used to live in the city." On the other hand, "be used to" is used to indicate being familiar with or accustomed to something. For instance, "She is used to the cold weather now." So, "used to" refers to past situations, while "be used to" refers to being familiar with or accustomed to something.
Notice that for the negative and interrogative sentences, the verb is in present, the auxiliary verb did indicate past and as a result there is no d.
The structure of "used to" is used to talk about past habits, states, or facts that were true before but are not true now. It is a modal verb that is always used in the past tense and is always followed by a base verb. For example, "I used to live in the city."
It's used to talk about things we did in the past, but now we don't do any more.
Notice that you can use the structure in different times and also that your sentence can be finished with a pronoun, object or verb-ing.
Things we are accustomed to; Things we are familiar with (other people may not be).
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