the penny drops

US /ðə ˈpɛni drɑps/
UK /ðə ˈpɛni drɑps/
"the penny drops" picture
1.

그제야 이해하다, 깨닫다

used to say that someone has finally understood something, especially after a period of not understanding

:
It took him a while, but then the penny dropped and he realized his mistake.
그는 시간이 좀 걸렸지만, 그제야 이해하고 자신의 실수를 깨달았다.
I kept explaining, but it wasn't until I drew a diagram that the penny dropped.
계속 설명했지만, 그림을 그리고 나서야 그제야 이해했다.