Definition (时间)
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会成为明星。
The second I saw her, I knew something was wrong.
我一看到她就知道有事情不对劲。
He'd been perfectly happy the whole time, and then the second his mother walked through the door, he started crying.
他一直都很开心，然后他妈妈一进门，他就开始哭了。