Sorgen er tung.

Because sorgen is the definite form of sorg.

• sorg = sorrow / grief in a general or indefinite sense
• sorgen = the sorrow / the grief

In Norwegian, the idea of the is often added as a suffix to the noun instead of being a separate word. So the -en at the end is doing the job that the would do in English.

In Bokmål, sorg can be treated as either masculine/common or feminine, depending on style and speaker.

That means you may see:

• en sorg → sorgen
• ei sorg → sorga

For learners, en sorg / sorgen is very common and completely standard, so Sorgen er tung is a very natural form to learn.

Because the adjective has to agree with the noun’s gender and number.

Here, sorgen is singular and not neuter, so the adjective is tung.

Compare:

• en stol er tung = a chair is heavy
• et bord er tungt = a table is heavy
• stolene er tunge = the chairs are heavy

So in Sorgen er tung, tung is the correct singular non-neuter form.

Because tung is a predicative adjective here: it comes after er and describes the subject.

In Norwegian, predicative adjectives usually agree with gender and number, but not with definiteness.

So you get:

• Sorgen er tung
• Bordet er tungt
• Steinene er tunge

But if the adjective comes before a definite noun, then you normally use the -e form:

• den tunge sorgen = the heavy sorrow

So:

• Sorgen er tung is correct
• den tunge sorgen is also correct

They are just different structures.

Er is the present tense of å være, which means to be.

So:

• å være = to be
• er = is / am / are depending on context

In this sentence, it simply means is.

It is most naturally figurative here.

Norwegian, like English, often uses words meaning heavy to talk about emotional pain, burden, or difficulty. So Sorgen er tung suggests that the grief feels weighty, difficult, oppressive, or hard to carry.

It is a very natural poetic or emotional use of tung.

They are related, but not identical.

• sorg usually means sorrow or grief, often something deeper and more serious
• tristhet usually means sadness, which can be broader and less intense

So sorg often sounds heavier and more emotionally charged than tristhet.

Yes, you could, but it means something slightly different.

• Sorgen er tung points to a specific sorrow or sorrow already understood from context
• Sorg er tung is more like a general statement about grief as a concept

So the difference is a lot like:

• The sorrow is heavy
• Sorrow is heavy

Both can be possible, but they are not exactly the same.

It follows a very basic Norwegian pattern:

subject + verb + adjective

So here you have:

• Sorgen = subject
• er = verb
• tung = adjective

This is very similar to English The sorrow is heavy.

A rough learner-friendly guide would be:

SOR-gen er toong

A few notes:

• sorgen has two syllables: sor-gen
• the g is pronounced
• tung ends with -ng like in sing
• the vowel u in Norwegian is not exactly the same as English oo, so toong is only an approximation

The sentence is short and fairly easy to say, but the vowel in tung is the part that usually needs the most practice.