It's raining hard. You can also say, "It's raining heavily." Sometimes 'hard' can be an adverb. It is an accepted convention. To make things confusing, If you say, "It's hardly raining" that means it is raining very lightly, almost not raining. So, "It's raining hard," has the opposite meaning of "It's hardly raining." Isn't English crazy?

Hello 高枫, There are 3 alternatives: - It is raining hard . In this case "hard" is an adverb describing the verb "to rain" . How is it raining ? It is raining hard. - It is hard rain. "hard" here is an adjective describing the noun "rain". How is the rain? It is hard rain. -It is hardly raining . It means it is certainly not raining and it indicates a minimal amount of the rain. "hardly" is another adverb distinguishable from the adverb "hard". For example: I can hardly understand the difference between hard and hard in this context . 88:)

hardly means very little. hard means difficult, or heavily. it's raining hard = the rain is pooring outside = it is raining like crazy outside it's raining hardly = its barely raining = its sprinkling outside

You can say "It's raining cats and dogs."

i think it is better to say it is raining heavily