rain 可做动词或名词。作动词常用的是做不及物动词意为“下雨”。名词就是“雨”了。
例句：It rains a lot。
常常下雨。
It is raining outside。外面正在下雨。
这两句里的It都是指代天气 说天气的时候一般都用it作形式主语。
其次 rainy是形容词 用来修饰名词 一般和day连用。
译为“多雨的，下雨的”
Rainy weather brings blessed relief to hay fever victims.
雨天会给枯草热患者带来欣喜，减轻他们的痛苦。
The rainy season in the Andes normally starts in December.
安第斯山区的雨季通常在12月份来临。
Strange as it may seem, she actually prefers rainy days.
听上去也许很奇怪，她其实喜欢下雨天。
She usually goes to work by bike except on rainy days.
除了雨天之外，她总是骑自行车上班。
The camping supplies were dampened during the long rainy season.
野营用品在漫长的雨季里受潮了。
I'll put the rest in the bank for a rainy day.
我要把余款存入银行以备不时之需。
rain 本身可作名词和动词用，就是雨水或下雨的意思。
作为动词用时，根据人称和时态自然衍生出如 rains / raining / rained 这些变化。例如：
It usually rains in the afternoon in Bangkok. 曼谷大都在下午会下雨。
It is raining now. 现在正在下雨。
It rained for 2 hours last night. 昨夜下了两小时雨。
rainy 是 rain 的形容词，表示“多雨的”
Summer usually is the rainy season in Beijing. 北京在夏季一般是多雨的雨季。
例句：
A. 动词: rain; rains; raining; rained
1. I think it will rain today. 今天会下雨[将来时]
2. It rains a lot in Hongkong [现在时]; It is raining now...[现在进行时]
3. It rained the whole weekend. [过去时]
4. "rain"的另一个意思:Rockets, mortars and artillery rained on buildings [枪弹如雨淋...]
B. 名词: rain作抽象的物质名词是不可数名词
Rain is much needed.非常需要雨。
Rain fell at intervals throughout night.
雨断断续续地下了一整夜。
You'll get soaked standing out in the rain.在雨中
你站在外面雨中会湿透的。
rain 一场雨，一阵雨，一次降雨；或者用作比喻义雨点般的某物;是可数名词
rains 大阵雨，雨季
A light rain began to fall.
小雨开始飘落。
Dark clouds are the sign of heavy rain.
乌云是大雨将至的预兆。
A rain of arrow fell on the soldiers.
雨点般的箭落在战士们身上。
a rain of tears(bullets, questions)
雨点般的泪水（子弹，问题）
There was no rain yesterday. 昨天没有下雨
We had flooding because of last week's heavy rains. 大雨[复数]
因为大阵雨我们发洪灾了。
The rains have started early this year.今年雨季来的早。
The rainy season is backward this year.
今年雨季来得晚。