This is the 19302nd most frequent Korean word.
않았을까
"Wouldn't have" or "might not have."
In this sentence, '않았을까' is used to express a reflective or speculative mood about the possibility of an action's negative occurrence.
그가 오지 않았을까 생각했다.
I wondered if he hadn't come.
Here, '않았을까' indicates concern about an unlikely scenario regarding a past event.
비가 오지 않았을까 걱정했어요.
I was worried that it might not have rained.
In this case, '않았을까' raises a speculative thought about the inverse degree of difficulty of the test.