Viruset er farlig.

Because Norwegian usually puts the definite article at the end of the noun instead of using a separate word like the.

• virus = a virus / virus
• viruset = the virus

Here, virus is a neuter noun in Bokmål, so the definite singular ending is -et.

So:

• et virus = a virus
• viruset = the virus

In Norwegian, definiteness is often shown by a suffix attached to the noun.

English says:

• the virus

Norwegian usually says:

• viruset

That -et ending already means the, so you do not add another word before it in a basic sentence like this.

Er means is / am / are, depending on context. It is the present tense of the verb å være (to be).

In this sentence:

• Viruset er farlig = The virus is dangerous

Some other examples:

• Jeg er trøtt = I am tired
• Han er lærer = He is a teacher
• De er hjemme = They are at home

Farlig is an adjective. It describes the noun.

So in:

• Viruset er farlig

the adjective farlig tells us what the virus is like: dangerous.

This is called a predicative adjective, because it comes after er rather than directly before the noun.

Compare:

• et farlig virus = a dangerous virus
• Viruset er farlig = The virus is dangerous

That is a very common question. Many Norwegian adjectives normally take -t in the neuter singular, but adjectives ending in -ig usually do not add another -t.

So:

• en farlig sykdom = a dangerous disease
• et farlig virus = a dangerous virus
• Viruset er farlig = The virus is dangerous

You do not say farligt in modern Norwegian Bokmål.

Yes. Although it stays farlig in the singular here, it changes in the plural and in some other contexts.

Common forms:

• farlig = singular, common use
• farlige = plural / definite / some other agreeing contexts

Examples:

• et farlig virus = a dangerous virus
• farlige virus = dangerous viruses
• Virusene er farlige = The viruses are dangerous

So farlig does inflect, just not with -t here.

Because this is a normal Norwegian statement sentence, and the basic word order is:

subject + verb + complement

So here:

• Viruset = subject
• er = verb
• farlig = adjective/complement

This is very similar to English:

• The virus is dangerous

Yes, but the meaning changes.

• Viruset er farlig = The virus is dangerous
  → a specific virus

• Et virus er farlig = A virus is dangerous
  → one virus / a virus in general, depending on context

So the difference is mainly definite versus indefinite.

No, not by itself.

After er, when you use an adjective like farlig, you usually just use the adjective alone:

• Viruset er farlig

You would only use en if a noun follows:

• Viruset er en farlig trussel = The virus is a dangerous threat

So:

• Viruset er farlig ✅
• Viruset er en farlig ❌

Yes, in Bokmål it is normally treated as a neuter noun:

• et virus
• viruset

That is why the definite singular form is -et.

A simple learner-friendly approximation is:

• VEE-roo-set er FAR-lee

A few notes:

• viruset has stress near the beginning: VI-ruset
• er is often pronounced very lightly in natural speech
• farlig is roughly FAR-lee, with the final g often weak or not fully pronounced in everyday speech

Exact pronunciation varies by dialect, but that approximation will help you get started.

This sentence is standard Bokmål.

In Nynorsk, you would more naturally see:

• Viruset er farleg

So the difference is mainly the adjective:

• Bokmål: farlig
• Nynorsk: farleg

Yes, very often virus has the same form in the indefinite singular and indefinite plural.

Examples:

• et virus = a virus
• flere virus = several viruses

The definite plural is:

• virusene = the viruses

So context tells you whether virus is singular or plural when it appears without an ending.