-더니
Use -더니 after an action or observed event when the next part shows a result or a change the speaker noticed. It often means something like 'after doing/seeing X, Y happened.' You can reuse it in everyday narration. Example: '집에 들어갔더니 불이 켜져 있었다.' = 'When I went into the house, the lights were on.' The story uses this pattern in '문을 열고 들어갔더니 음악이 잔잔하게 흐르고 있었다.'.
집에 들어갔더니 불이 켜져 있었다.
When I went into the house, the lights were on.
