A policeman has to keep on his toes all the time he's on duty.

警察在执勤时必须时刻警覺。

Athletes have to train continuously to stay in peak condition.

运动员必须不断训练才最佳竞技状态。

On this superhighway drivers should keep to a speed of over 80 miles per hour.

在这条高公路,驾驶员必须每小时80英里以度。