keep flat

例句：

1. 1. The car was parked on the flat surface of the road.
   汽车停在了道路的平坦表面上。
2. 2. The roof needs to be kept flat to prevent leaks.
   屋顶需要保持平坦以防止漏水。
3. 3.The floor is kept flat to ensure a smooth walking experience.
   地板被保持平坦以确保行走时无障碍。