Sykdommen er alvorlig.

Because -en is the definite singular ending for many common-gender nouns in Norwegian.

• en sykdom = a disease / an illness
• sykdommen = the disease / the illness

So instead of using a separate word like English the, Norwegian often adds the definiteness to the end of the noun.

In Norwegian, the definite article is usually attached to the noun as a suffix.

So:

• en sykdom = a disease
• sykdommen = the disease

That means sykdommen already includes the idea of the, so you do not say the sykdommen or anything similar.

The base form is sykdom.

Its forms are:

• en sykdom = a disease
• sykdommen = the disease
• sykdommer = diseases
• sykdommene = the diseases

A learner will often want to recognize the dictionary form, and here that form is sykdom.

Er means is / am / are. It is the present tense of the verb å være = to be.

Examples:

• jeg er = I am
• du er = you are
• sykdommen er = the disease is

Unlike English, Norwegian does not change the present-tense form for different persons. So er is used with all subjects in the present tense.

Alvorlig is an adjective. It means serious (and sometimes severe, depending on context).

In this sentence, it describes sykdommen:

• Sykdommen = the disease
• er alvorlig = is serious

So the full sentence means The disease is serious.

Because this is a very common pattern in Norwegian:

• subject + verb + adjective

So:

• Sykdommen = subject
• er = verb
• alvorlig = adjective describing the subject

This is similar to English:

• The disease is serious

The adjective comes after to be / være when it describes the subject.

In this sentence, it stays alvorlig.

Norwegian adjectives can change form in some situations, but here the adjective is being used after er as a predicate adjective, and alvorlig is the correct form.

You may also see:

• en alvorlig sykdom = a serious disease
• sykdommen er alvorlig = the disease is serious
• alvorlige sykdommer = serious diseases

So yes, adjectives can change, but in this sentence alvorlig is exactly what you expect.

Yes, in this sentence it is very similar to English.

• Sykdommen = The disease
• er = is
• alvorlig = serious

So the order is:

• Subject + Verb + Complement

That gives:

• Sykdommen er alvorlig.
• The disease is serious.

This is a nice beginner-friendly sentence because the structure matches English quite closely.

You could, but it would mean something different.

• Sykdommen er alvorlig = The disease is serious
  → talking about a specific disease

• Sykdom er alvorlig = Disease/Illness is serious
  → more general, talking about disease as a concept

So if you mean one particular disease, sykdommen is the right choice.

In Bokmål, sykdom is usually treated as a common-gender noun:

• en sykdom
• sykdommen

That is why the definite ending is -en.

For practical learning, the important thing is to memorize it with its article:

• en sykdom

A rough guide is:

• SYK-dom-men

A few pronunciation notes:

• The y in syk- is not like English y in yes.
  It is a rounded vowel, somewhat like German ü or French u.
• The kj/sj-like sound is not here; the beginning is just s + y + k.
• The stress is usually on the first syllable: SYK-

A rough English-friendly approximation is something like süük-dom-men, but that is only approximate.

A rough guide is:

• AHL-vor-li

Some helpful notes:

• The g at the end is often weak or not pronounced strongly in normal speech.
• The stress is usually on the first syllable: AHL-
• The r is Norwegian, so it may sound different from an English r depending on dialect.

A very rough approximation is AHL-vor-lee, but native pronunciation will vary a bit.

Its most common meaning is serious, but depending on context it can also mean something like severe or grave.

For example:

• en alvorlig sykdom = a serious disease
• en alvorlig feil = a serious mistake
• situasjonen er alvorlig = the situation is serious

So alvorlig is a useful general adjective, not just a medical word.

This sentence is Bokmål:

• Sykdommen er alvorlig.

In Nynorsk, you would more likely see:

• Sjukdomen er alvorleg.

So if you are studying Bokmål, the given sentence is the normal form to learn.

Yes, but then the structure changes:

• en alvorlig sykdom = a serious disease

That is different from:

• Sykdommen er alvorlig = The disease is serious

So Norwegian allows both patterns, just like English:

• a serious disease
• the disease is serious

Because det is not needed here.

A simple sentence with a noun subject often just uses:

• Subject + er + adjective

So:

• Sykdommen er alvorlig

You use det in other kinds of sentences, for example with weather, general statements, or certain clause structures, but not here.

Yes. It is a full, natural Norwegian sentence.

It has everything it needs:

• Sykdommen = subject
• er = verb
• alvorlig = description/complement

So even though it is short, it is grammatically complete and perfectly normal.