Modal verbs
Used
+ infinitive
We use
used
+
infinitive
to talk about finished past states or habits.
I
used to live
here when I was a child.
(state)
They
used to fight
a lot, but now they don't.
(habit)
+
I/you/we/they/she/he/it
used
to smoke
.(?)
-
I/you/we/they/she/he/it
didn't
use
?
Did
I/you/we/they/she/he/it
use