The soldiers need to be on the alert at all times.士兵们需要时刻保持警惕。

When you're driving in winter you should always be on the alert for icy conditions.冬天驾车时你应该时刻警惕出现路面结冰的情况。

I'm always on the alert for a good bargain.我一直留意着淘件便宜货。