This grammar construct shows curiosity or doubt about a past fact/happening.

- always used in the past tense
- interchangeable with 는가 했다/은가 했다/ㄴ가 했다 with no difference in meaning

Examples:
누가 왔나 했어요./I wondered who came.
그 책이 어디 있나 했지요./I wondered where the book was.