請看劍橋英語英英詞典對名詞rain的解釋:
drops of water from clouds
examples:
Rain is forecast for tomorrow.
Come inside out of the rain.
We had heavy/light rain all day.(註意這個,並沒有加a)
We got caught in pouring/torrential (= a lot of) rain without either raincoats or umbrellas.
There will be showers of rain/rain showers (= short periods of rain) in the east.
It looks like rain (= as if rain is going to fall).
cloud是可當可數名詞,也可當不可數名詞,這裏的the clouds就是表示雲的復數形式
2.it 指代上句的the vapour,(特指)因為是第三人稱單數所以 come from要加s。It comes from the water 是典型的主謂賓的句型,in the river做the water的後置定語,修飾the water。
3.像1.講到的那樣,這裏的cloud是不可數名詞,所以用單數形式
補充:
the 加在名詞前表示特指或者壹類事物!這裏沒有強調特指所以用的是vpour的不可數形式!