This verb "use" in this odd usage as an auxiliary verb creating a past tense is very odd, because it seems to have lost its present tense form (which would be "use" or "uses"), but it still has an infinitive form, which you can use with "did" to form questions ("Did you use to live in London?") and negatives ("I didn't use to live in London"). It also lacks the participles (which would be "using" and "used") with this meaning as well. However, it's not really a deficient verb, because "use" has other meanings where it has the full range of tenses and forms.
I think it's interesting that they have the same usage of "used to" in Swedish as we do in English. They use their own verb "to use." It was hard to get in the habit of using it, because I always felt like I was translating too literally from English.
Oh and I agree with shilgia, I think secretcover has misunderstood the specific point being made.

i suppose the hitch with the constructions is that it is one that is only present on paper. phonetically, "did he use to" and "did he used to" sound the same as far as the d is concerned since it is absorbed by the t ...then comes the z problem-

Shilqia, I looked it up in the Oxford. You are right and I am wrong. Seems one is never too old to learn.

I'm not the only one who thinks of the current situation in the US Justice Department when the phrase "fall guy" comes up. (See #1.)
The acting associate attorney general just withdrew his name from consideration for the permanent position and resigend the acting position, just before he would have faced questioning from the Senate Judiciary Committee on his appointment. Senator Charles Schumer of that committee commented "The Attorney General is running out of fall guys."