$\text{I live in a flat when I was a child.}$
$\text{⇒ Answer:}$
$\text{+) I used to live in a flat when I was a child.}$
$\text{-) I didn't used to live in a flat when I was a child.}$
$\text{?) Did you use to live in a flat when you was a child?}$
$\text{Tạm dịch: Mình sống trong một căn hộ khi mình còn là một đứa trẻ.}$