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

警察在必须刻保持警覺。

Athletes have to train continuously to stay in peak condition.

运员必须不断训练才能保持最佳竞技状态。

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

在这条高速公路,驾驶员必须保持每小80英里以的速度。