rain 可作动词也可做名词
－动词：It will rain today.今天会下雨
—名字：The rain is smaller now.现在雨下小了.
raining是rain作为动词的进行时态
It is raining heavily outside.现在外面雨很大.
rainy是形容词
It is a rainy day.今天是个雨天
估计初中的填空选择会这样出题：
I can go swimming with you tomorrow because the weather report said it will _____.（这里填rain,为动词）
I can go swimming with you tomorrow because the weather report said it will be a ____ day.(这里填rainy.为形容词)
I can go swimming with you today because it is _____ outside.(这里填raining,动词进行时态)