Definition (時間)
Immediately after something happens; at the very moment that.
立刻
The second I saw him, I knew he was going to be a star.
我一看到他就知道他會成為明星。
The second I saw her, I knew something was wrong.
我一看到她就知道有事情不對勁。
He'd been perfectly happy the whole time, and then the second his mother walked through the door, he started crying.
他一直都很開心，然後他媽媽一進門，他就開始哭了。