Negative form

Subject

Negative form

Past habit

Verb simple form

Complement

I

You

He / She / It

We

They

did not (didn’t)

use to

spend

a lot of money.

Interrogative form

Auxiliary Did

Subject

Past habit

Verb simple form

Complement?

Did

I

you

he / she / it

we

they

use to

spend

a lot of money?

Short answers

Affirmative answers

Negative answers

Yes,

I

you

he

she
it

we

you

they

did.

No,

I

you

he

she
it

we

you

they

didn’t.

More examples:

(affirmative)
David used to have very long hair when he was 15 years old.  
(negative)
David didn’t use to have very long hair when he was 15 years old.
(interrogative)
Did David use to have very long hair when he was 15 years old?
(Aff. answer)
Yes, he did.  
(Neg. answer)
No, he didn’t.