Regnet er kraftig.

Regn means rain as a general substance or idea.

Regnet is the definite singular form, literally the rain.

In Norwegian, it is very common to use the definite form when talking about the rain that is happening now or a specific period of rain.

Compare:

• Regn er vanlig om høsten. = Rain is common in autumn.
• Regnet er kraftig. = The rain is heavy / The rain is intense.

So regnet here refers to a particular rain situation, not rain in general.

Because regn is a neuter noun in Norwegian.

A lot of neuter nouns form the definite singular by adding -et:

• et hus → huset
• et brev → brevet
• et regn → regnet

If a noun is common gender, it usually takes -en instead:

• en bil → bilen

So regnet is correct, while regnen is not standard Bokmål.

Because kraftig comes after er, so it is a predicative adjective.

In this position, the adjective agrees with gender and number, but it does not take the same definite ending you would see before a definite noun.

Also, adjectives ending in -ig usually have the same form in common gender and neuter singular:

• en kraftig vind
• et kraftig regn
• Regnet er kraftig

You use kraftige in forms like:

• det kraftige regnet = the heavy rain
• kraftige stormer = strong storms

So Regnet er kraftig is exactly what you expect grammatically.

Its core meaning is something like strong, powerful, intense, or substantial.

With rain, the most natural English translation is usually heavy:

• kraftig regn = heavy rain

But in other contexts, kraftig can mean different things:

• kraftig kaffe = strong coffee
• kraftig motor = powerful engine
• kraftig vekst = strong growth

So the exact English word depends on the context.

Yes, absolutely.

In fact, Det regner kraftig is often the more natural everyday way to say it.

The difference is mainly one of focus:

• Det regner kraftig. = It is raining heavily.
  
  • Focus on the action of raining.
• Regnet er kraftig. = The rain is heavy/intense.
  
  • Focus on the rain itself as a thing or condition.

Both are correct, but Det regner kraftig often sounds more conversational.

Because Norwegian usually puts the definite article at the end of the noun as a suffix.

So instead of a separate word like English the rain, Norwegian often says:

• regnet = the rain

This is very normal in Scandinavian languages.

However, if there is an adjective before a definite noun, Norwegian usually adds a separate article too:

• regnet = the rain
• det kraftige regnet = the heavy rain

So in your sentence, there is no extra article because the noun stands alone and already has the definite ending -et.

You would say:

det kraftige regnet

This shows a very important Norwegian pattern called double definiteness:

• det = definite article
• kraftige = adjective in definite form
• regnet = noun in definite form

Compare:

• et kraftig regn = a heavy rain
• det kraftige regnet = the heavy rain

So:

• Regnet er kraftig. = The rain is heavy.
• Det kraftige regnet = The heavy rain.

Usually, kraftig is the more natural choice with rain.

Norwegian has preferred adjective-noun combinations, just like English does. For rain, kraftig regn is the standard expression for heavy rain.

Compare:

• kraftig regn = heavy rain
• sterk vind = strong wind

So even though sterk and kraftig can both relate to strength, they are not always interchangeable. With regn, kraftig is the normal and idiomatic choice.