used to

This is a well-known question about which there seems no universal agreement. In writing, both look pretty horrible to me. In speech they sound the same, so people may not be clear about which they are saying. For me, the actual proper way to say this is "Used you to smoke?", but unfortunately this now sounds too stilted for everyday use.

Swan prefers the second one, but does not deny the validity of the first one.

I used to smoke. Did you use to smoke? [ The past tense indicator is in the auxiliary verb did.
[ I made this very mistake yesterday in a quickly-written email. Had I taken the time to proofread, the error would have stood out like a sore thumb. The cause: used to and use to are pronounced nearly exactly the same. ]

I don't perceive "used" in "I used to smoke" as an inflected form of the verb "use". I suppose it was by derivation, but now it seems entirely idiomatic to me. Therefore, I do not see any reason to "uninflect" it and create "Did you use to smoke?"

Michael Swan says:When questions and negatives are written, they often have did.....used instead of did.... use.
What did people use(d) to do in the evenings before TV?
The contraction usedn't is also possible. I usedn't to like opera.
But the most common negative is never used.... I never used to like opera
In a formal style,questions, and negatives without do are possible,but

nazanin saryazdiBut the most common negative is never used.... I never used to like opera

Hmm. "I never used to like ...." is very common around here.

nazanin saryazdiThis is all confusing

Yes, when you read about all the theoretical possibilities, it is confusing. For practical use, I recommend these:

I used t

Jim!!! I wish you were my teacher!! Thank you.

In writing, I prefer "used not to" to "didn't use(d) to".

In fact, make that in speech also.

nazanin saryazdiThe contraction usedn't is also possible. I usedn't to like opera.

I didn't know that Michael Swan was Czech.

GPYIn writing, I prefer "used not to" to "didn't use(d) to".In fact, make that in speech also.

You probably make more money than I do.