The first two sentences describe the calm of the pond water surface after the rain, only vaguely showing the word "flat", followed by the realm of "light grinding", showing the quiet beauty of spring scenery on the pond after the rain.
The last two sentences show a kind of dynamic beauty on the pond after rain, such as "sudden rise", "weeping poplar dancing" and "ten thousand points" sound made by raindrops on weeping poplar leaves blown into lotus leaves by the wind.