keep guard over

例句：

1. 1. The soldiers kept guard over the border.
   士兵们守护着边境。
2. 2.The policeman was keeping guard over the crowd.
   警察在人群中守护着。
3. 3. She kept guard over her children while they were sleeping.
   在她孩子们睡觉时，她守护着他们。