뭘 했는지

what someone did

→ 뭐+를
phrase Rare #29,232
what someone did Used to refer to what someone did, often as something to find out, know, or understand. 뭘 is a contracted form of 무엇을.
KO: 어제 뭘 했는지 말해 줘.
EN: Tell me what you did yesterday.

차 안에서 뭘 했는지 다 알아요!

I know what you did in the car!

— 필사의 추적 (1981)

그 사람이 남긴 쓰레기를 보면 최근에 뭘 했는지 알 수 있다

You can tell what someone has done recently by looking at the trash they left behind.

— 타락천사 (1995)

내 전화를 끊고 로이스가 뭘 했는지 아나?

After I hung up, you know what Lois did?

— 좋은 친구들 (1990)

내가 셰이퍼 학교에서 뭘 했는지 사람들은 이해를 못 하는 것 같아

I don't think people understand what I did at Shaper School.

— 위플래쉬 (2014)