why does clound not drop on the ground 云为什么会浮在天上，而不掉到地上来

IT does, and it is called fog when it is at ground level.

Yes I suppose that is how water falls from a cloud. Fog is essentially the same thing as a cloud, but formed at ground level.

So I'm a bit confused about what your question was asking (if you know it falls as rain)?

Hi, I know the answer, it's easy, the small drops of water that have in the fog fall to the ground when it condenses too much, increasing his weigh and the ascendent forces as the pressure of the air can't support the weigh of the water.