复合介词in case of
1. 表示条件，意为“如果”“万一”。如：
In case of fire, ring the bell. 如发生火灾，就按铃。
In case of emergency, break the glass and press the button. 遇到紧急情况时，击碎玻璃罩并按下按钮。
In case of my not being here, ask my brother to help you. 如果我不在可让我弟弟帮助你。
2. 表示目的，意为“以防”。如：
I keep an umbrella here in case of rain. 我在这里放一把雨伞以防下雨。
The wall was built along the river in case of floods. 沿江筑了堤坝，以防涨水。