보낸 건

the fact that (someone) sent

→ 보내다
phrase Rare #26,645
the fact that (someone) sent 건 is a contraction of 것은. This pattern nominalizes an action and presents it as the topic; the verb form can change, as in 보내는 건, 보낼 건, or 보내신 건.
KO: 네가 그 편지를 보낸 건 알고 있어.
EN: I know that you sent that letter.

스페인 난민 에게 돈을 보낸 건 압니다

I know you sent money to Spanish refugees.

— 오펜하이머 (2023)