아니었을까

In a sentence

• 그가 더 열심히 공부했다면 합격하지 않았을까 싶다.

  If he had studied harder, he might have passed.

  In this sentence, '합격하지 않았을까' implies a speculation or doubt about the possibility of passing.

• 이 길이 아니라면 아마도 잃어버린 물건을 찾지 않았을까?

  If not this path, might we have already found the lost item?

  Here, '찾지 않았을까' questions an alternative possible conclusion in a hypothetical situation.

• 그날 시간이 있었다면 그냥 집에 가지 않았을까 추측해 본다.

  If they had time that day, I guess they would have just gone home.

  '가지 않았을까' speculates about what might have happened given a past condition.