들었을

In a sentence

• 나는 어제 친구의 이야기를 들었을 때 매우 놀랐어요.

  I was very surprised when I heard my friend's story yesterday.

  The word '들었을' is used as the past tense form of '듣다' (to hear), functioning within a time clause to set the timing of the main action, '놀랐어요' (was surprised).

• 그 소식을 들었을 경우, 당신은 어떻게 하겠습니까?

  If you hear the news, what would you do?

  Here, '들었을 경우' introduces a conditional clause with '들었을' indicating a hypothetical past hearing scenario acting as a setup for the potential reaction

• 음악을 들었을 때 기분이 정말 좋아졌어요.

  When I listened to music, my mood really improved.

  In this sentence '들었을' again reflects a past action, here specifically associated with experiencing an event, '들었을 때' (when listened to).