刑務所にいた

was in prison

→ 刑務所
phrase Rare #35,306
was in prison Literally means “was at/in prison.” いた is the past tense of いる, used here for being located in prison or serving time there.
JA: 彼は若いころ、刑務所にいた。
EN: He was in prison when he was young.

彼は死ぬまで刑務所にいた

He was in prison until he died.

刑務所にいたんですか

Were you in prison?

— ミスティック・リバー (2003)

彼は自分が刑務所にいたことを私に明らかにした

He revealed to me that he had been in prison.

刑務所にいたんですよ

I was in prison.

— 親切なクムジャさん (2005)

私が刑務所にいたことトムメアリー言ってないといいんだけど

I hope Tom hasn't told Mary that I was in prison.