About the usage of "Used to"

source: BBC Learning English

[B]'Used to' or 'would' in Learn it today[/COLOR][/B]
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Julie from Belgium writes:

Is this construction correct: Did you used to play tennis?

Ehtisham Haq, studying English in the US, writes:

I would like to know the correct use of used to and would for past habits. Please tell me which of these is correct:

I used to live in that house.
I would live in that house.
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Roger Woodham replies:  
      
used to: questions and negative forms[/COLOR]

Used to is used to describe past habits or long-lasting actions and situations which are now finished

[INDENT]* People used to think[/COLOR] the sun revolved around the earth.
* I used to take[/COLOR] size 12, but now I take size 14.[/FONT][/INDENT]
For questions and negative forms, two forms of the verb are used - either the normal infinitive pattern after did (more common), or the past form used (less common):

[INDENT]* When you were a kid, did you use[/COLOR] to think the sun revolved around the earth?
* When you were a kid, did you used[/COLOR] to think the sun revolved around the earth?
* I didn't use[/COLOR] to take such a large dress size, but now I do.
* I didn't used[/COLOR] to take such a large dress size, but now I do.[/FONT][/INDENT]
In a more formal , questions and negatives are possible without do, following the pattern of a modal auxiliary verb, although these forms are less often used:

[INDENT]* I used not to like[/COLOR] contemporary dance, but now I do.
* Used[/COLOR] you to play[/COLOR] the organ in church before you became a monk?[/FONT] [/INDENT]  
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in 1996/last month etc. - usually/frequently/often etc.

When we use used to, we are describing things that happened at an earlier stage in our lives which are no longer in place as circumstances have changed. Note that if we want simply to refer to what happened in the past, we normally use the simple past tense, often with an adverbial time phrase:

[INDENT]* From 1995 to 1998[/COLOR] I lived[/COLOR] in that house and then[/COLOR] I emigrated[/COLOR] to Australia.
* I returned[/COLOR] to Britain two years ago[/COLOR] and last year[/COLOR] I bought[/COLOR] this house in Bath.[/FONT][/INDENT]
Note that when we want to talk about present habits and states, we use the present simple tense, often with an adverb of frequency:

[INDENT]* I usually do[/COLOR] my homework immediately after supper.
* I occasionally smoke[/COLOR] cigars, but never cigarettes.
* I normally use[/COLOR] public transport in London, but I sometimes drive in despite the congestion charge.[/FONT][/INDENT]
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would or used to?

When we are telling a story and recollecting an event from long ago, we often prefer to use would[/COLOR] to describe repeated behaviour in the past, although both would[/COLOR] and used to[/COLOR] are possible:

[INDENT]Do you remember what we used to get up to when we were teenagers? How I would wait for you nearly every afternoon after school and then we would stroll home together across the park, holding hands, and you would feed the ducks on the pond while I had a cigarette?[/FONT][/INDENT]
Note, Ehtisham, that would[/COLOR] in this sense describes past events and actions[/COLOR]. It cannot be used to refer to past states[/COLOR]. To describe past states[/COLOR] we can only use used to[/COLOR]:

[INDENT]* I used to live[/COLOR] in that house over there. (NOT: I would live in that house over there.)
* I used to own[/COLOR] a 1966 Silver Cloud Rolls Royce. (NOT I would own a 1966 Silver Cloud Rolls Royce.)
* I used[/COLOR] sometimes to drive[/COLOR] to work in it.
* I would[/COLOR] sometimes drive[/COLOR] to work in it.[/FONT][/INDENT]

used to

Used to expresses the idea of something we did in the past but no longer do: When I was a child, we used to go to Scarborough for our holidays.

Using used to in questions and negative sentences can present problems. The usual way of turning a sentence like He used to snore into a question is with the word did. This is straightforward in spoken English, but there are two possible ways of writing it. The more logical is: Did he use to snore? The alternative, Did he used to snore?, is becoming more accepted, but it still strikes many people as odd. You can also make a question by reversing the word order: Used he to snore? But this is becoming less common.

The usual way of making used to negative is with didn't. But again, there's a problem with how to write it. He didn't use to snore is more widely acceptable than He didn't used to snore. You can also put not after used, although this is becoming less common: He used not to snore. The contracted written form is usedn't, not usen't. You can avoid any difficulty by using never: He never used to snore.

For negative questions, you can say: Didn't he use (or used) to snore? or Usedn't he to snore? (the uncontracted form of this, Used he not to snore?, is rather pompous and old-fashioned).