Used to is used for a past habit that doesn't happen anymore. For example:
- I used to play soccer at weekends. (But not now)
- He used to get up at 5.30, but now he gets up at 8.00.

Used to is also used for past situations and facts. For example:
- People used to think the world was square.
- He used to be a film director.

Used to is used only in the past simple. For example:
- We used to work together. OK
- We use to work together. Incorrect

Used to can be used in questions and negatives like any other verb. For example:
- I didn't use to exercise regularly.
- Did you use to work here?
Note that the use of did and the base verb use (without a "d" at the end) is the same as normal question and negative formation.

Used to is a completely different structure from be used to. Used to is for past habit, be used to means to get accustomed to something.

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