旧 (Kanji for former)

PREFIX meaning 'former.' It can be used with lots of different words, but unfortunately none of them are really super-handy. For example . . .

Onyomi

KYUU

One day I will re-unite with my CUTE former girlfriend.

旧姓(きゅうせい)	maiden name ★☆☆☆☆ 旧 (former) + 姓 (family name) = 旧姓 (maiden name)
復旧(ふっきゅう) する	to resume service ☆☆☆☆☆ 復 (return or re-do) + 旧 (former) = 復旧 (to resume service)