The structure used to + the infinitive is sometimes confusing for learners of English. Here is an explanation with examples of use.

AFFIRMATIVE	NEGATIVE		INTERROGATIVE
	Long Form	Contracted Form
I used to smoke	I did not use to smoke	I didn't use to smoke	Did I use to smoke?
You used to smoke	You did not use to smoke	You didn't use to smoke	Did you use to smoke?
He/she used to smoke	He/she did not use to smoke	He/she didn't use to smoke	Did he/she use to smoke?
We used to smoke	We did not use to smoke	We didn't use to smoke	Did we use to smoke?
You used to smoke	You did not use to smoke	You didn't use to smoke	Did you use to smoke?
They used to smoke	They did not use to smoke	They didn't use to smoke	Did they use to smoke?

The structure used to + infinitive is used to refer to a past habit or situation which no longer exists at the present time.

• It refers to past habits and states that do not exist today, something that you did regularly or repeatedly in the past but no longer do today.
  
  
• It exists only in the past.
  
  Examples :
  
  
  • Tom used to drink coffee. Now he prefers tea.
  • Julie used to fly from London to Paris. Now she takes the Eurostar.
  • I used to drive to work. Now I take the underground.
  • Bill used to live in Wales. Now he lives in Scotland.
  • Wendy used to eat meat. Now she's a vegetarian.
  • Eva used to wear glasses. Now she wears contact lenses.
  • My mother didn't use to do much exercise. Now she goes walking with a friend.
  • My grandfather didn't use to approve of social networks.
    Now he uses them every day to stay in contact with his grandchildren!
    
    
• This structure cannot be used to say how often something happened, or how long it took.
  
  
  • Julie used to visit Paris seven times. Julie visited Paris seven times.
  • Bill used to live in Wales for three years. Bill lived in Wales for three years.

N.B.  Do not to confuse 'used to' with 'to be/to get used to'.

See also: 'would vs 'used to'