Sykdommen er dødelig.

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

• sykdom = disease / illness
• sykdommen = the disease

So instead of putting a separate word like the in front, Norwegian often adds the definiteness to the end of the noun.

In Norwegian, the is usually built into the noun as a suffix.

Compare:

• en sykdom = a disease
• sykdommen = the disease

This is very different from English, where the is a separate word.

er means is / am / are. It is the present tense of å være (to be).

So:

• Sykdommen er dødelig = The disease is deadly/fatal

A few useful forms of å være:

• er = am / is / are
• var = was / were
• har vært = have/has been

dødelig means deadly, fatal, or lethal, depending on context.

In this sentence, it describes the disease as something that can cause death.

Related words:

• død = death / dead
• å dø = to die
• dødelig = deadly / fatal

So there is a clear connection to the idea of death.

Because the adjective is being used after the verb er, and the subject is singular.

In Norwegian, adjectives often change form depending on number and gender, but here the singular form is dødelig.

Compare:

• Sykdommen er dødelig = The disease is deadly
• Sykdommene er dødelige = The diseases are deadly

So dødelige would be used for plural.

In Bokmål, sykdom is usually treated as a masculine noun:

• en sykdom
• sykdommen

Some learners worry a lot about noun gender, but in this sentence the important thing is recognizing that -men here marks the disease.

It can cover both ideas, depending on context.

• disease is often a good translation in medical or formal contexts
• illness can also fit in some situations

In Sykdommen er dødelig, disease sounds like the most natural English translation.

The word order is very straightforward:

• Sykdommen = subject
• er = verb
• dødelig = adjective/complement

So it follows the pattern:

Subject + verb + complement

That is very similar to English:

The disease + is + deadly

A learner will often notice the vowels first, because Norwegian has sounds that English does not have.

A rough guide:

• sykdommen ≈ SÜK-dom-men
• dødelig ≈ DUR-de-lig or DUH-de-lig, with a rounded vowel

Two tricky letters:

• y is not like English y; it is a rounded front vowel
• ø is also a special Norwegian vowel, somewhat similar to the vowel in some British pronunciations of bird, but rounded

The exact pronunciation varies by accent, but those vowels are the main challenge.

You might be understood, but dødelig is the more natural everyday Norwegian word here.

• dødelig = the normal, common word for deadly/fatal
• fatal exists, but it is less basic and often feels more formal or less idiomatic in ordinary speech

So for a learner, Sykdommen er dødelig is the safest and most natural choice.

Yes, sometimes dødelig can be used more broadly, just like English deadly.

For example, in different contexts it might mean:

• literally fatal
• extremely dangerous
• in some informal expressions, even very intense

But in this sentence, the most natural interpretation is the literal one: the disease can cause death.

The indefinite version would be:

• En sykdom er dødelig.

That means A disease is deadly or One disease is deadly, depending on context.

Compare:

• en sykdom = a disease
• sykdommen = the disease

So the original sentence is talking about a specific disease.

You would change both the noun and the adjective:

• Sykdommene er dødelige. = The diseases are deadly.

Here:

• sykdommene = the diseases
• dødelige = plural adjective form

So this sentence is a good example of how Norwegian adjectives can show number.

This sentence is Bokmål.

In Nynorsk, you would more likely see:

• Sjukdomen er dødeleg.

So if you are learning Bokmål, Sykdommen er dødelig is the correct form to focus on.