雨是名词 下是动词，下雨动宾结构

雨 refers to the noun, whereas 下雨 refers to the verb (rain/raining)

巴山夜雨涨秋池 Bashan night rain rises autumn pool 可见“雨”、“下雨”有时都作名词。 一般情况下:雨、雨衣、雨鞋、雨伞是名词。下雨是动词。

In English, "rain" serves both a noun and verb! In modern Chinese, 雨 is a noun and 下 雨is the verb form, literally "falling rain". However, in Classical Chinese, 雨 functions as a noun or verb. For example, 今朝天清昨夜雨. Here it can be a noun or verb. 1. This morning's clarity is due to last night's rain. 2. This morning's clarity is because it rained last night.

You may want to picture it this way (if it helps): 1. 雨 as "the thousands of water droplets in the sky" 2. 下雨 as "the falling of these water droplets to the ground" Just a few examples (hopefully to clarify). 雨： 1. 雨下得很大。 The rain is pouring down hard. (literal translation) In other words, it's pouring, it's raining heavily. 下雨： 1. 下雨了。 It's raining. 2. 在下雨。 It's raining. 3. 昨天下雨了。 It rained yesterday. 4. 我下雨(的)天睡得比较好。 I sleep better when it rains. I sleep better on rainy days.