This pattern expresses while one was doing something, something was suddenly discovered.

- only used with action verbs
- the 가 part is optional
- the main clause of this pattern is always in the past tense

Examples:
이야기하가 보니, 열 한시가 되었어요./While I was talking, I realized it had become 11 o'clock.
소주를 마시다가 보니, 돈은 없어졌어요./While drinking soju, I realized I exhausted my money.