나는 그가

I ... he

→ 나
phrase Rare #14,360
I ... he This is not a fixed expression; it combines 나는 (“I”) with 그가 (“he,” as the subject of the following clause). The following verb or adjective completes the meaning.
KO: 나는 그가 좋은 사람이라고 생각해.
EN: I think he is a good person.

나는 그가 화난 줄 알았다.

I thought he was angry.

나는 그가 누군지 안다.

I know who he is.

나는 그가 아니야

I'm not him.

— 아바타: 물의 길 (2022)

나는 그가 내 형제 줄 알았다.

I thought he was my brother.