I used to smoke. (Je fumais.)
He didn't use_ to smoke. (Il ne fumait pas.)
Did you use_ to smoke? (Fumais-tu ?)

Notice that the expression used to is followed by a base verb, not a gerund. In negations and questions, the letter d is omitted from used to.

Remember, there is only one d. The use of the auxiliary verb did shows the past. Since the auxiliary verb did ends with d, there's no need to have a d on use to.

Notice that would can often be used instead of used to if the verb can be used in progressive tenses.

Use:

Used to is used to discuss past habits or states which are not true now. Among other things, the Imparfait tense expresses this in French.

Visually, this could be represented like this:

Misuse:

Do not confuse the Past Progressive and Used to. Used to is an idiom, not a tense.
Do not confuse Used to with be used to or get used to. These mean completely
different things.

There is no present or future equivalent of used to. Remember it is a past habit or
state. Do not use it for present habits or states.

Used To or Past Progressive

Many mistakes are made using the Past Progressive tense. With francophones, the
majority of these mistakes are caused by translating the French Imperfect tense. This is used for repetitive actions in the past. In English, we use used to for this purpose. Sometimes would can be used if it is a progressive verb (not a stative verb).

Remember that the Past Progressive is used in connection with another time or event.

It is incomplete by itself. If you use it without another time or event, people will
naturally ask, “... yes? and then what happened?” because they wonder what the
interruption was.

On my first job, I was making hamburgers.
On my first job, I used to make hamburgers.

Before I got a good job, I was borrowing from my MasterCard to pay my Visa.
Before I got a good job, I used to borrow from my MasterCard to pay my Visa.

People were writing more letters before we had e-mail.
People used to write more letters before we had e-mail.

Before Christopher Columbus, people were believing the world was flat.
Before Christopher Columbus, people used to believe the world was flat.

When I was ten, I was riding my bicycle to school.
When I was ten, I would ride my bicycle to school.