This is the 13393rd most frequent Korean word.
때렸던
"때렸던" means "had hit" or "struck" in the past tense.
In this sentence, '때렸던' (hit) is a past action used as an adjective to describe '기억' (memory). It shows the memory of an action that happened in the past.
나는 어릴 때 나쁜 친구를 때렸던 기억이 나.
I remember how I hit a bad friend when I was young.
Here, '때렸던' (hit) is used as a past participle to describe '사람' (person), indicating the person who was hit by '그' (he).
그가 때렸던 사람이 아직도 화가 나 있다.
The person he hit is still angry.
In this sentence, '때렸던' (hit) functions as a descriptor of '공' (ball), referring to the ball that was hit previously in the past.