Gassen er farlig.

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

• en gass = a gas
• gassen = the gas

So Gassen er farlig means The gas is dangerous, not just Gas is dangerous in a general sense.

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

Examples:

• Jeg er trøtt. = I am tired.
• Du er snill. = You are kind.
• Gassen er farlig. = The gas is dangerous.

Unlike English, Norwegian does not change the present form for different subjects. It is always er.

Farlig is the adjective meaning dangerous. It comes from fare, which is related to danger, plus the adjective ending -lig, which often works a bit like English -ly or -ous in word formation.

So:

• fare = danger
• farlig = dangerous

You do not need to analyze it every time, but it can help you recognize related words.

It stays farlig because it comes after er and describes the subject. This is called predicative adjective use.

In Norwegian, adjectives usually do not show gender or number agreement after verbs like er.

So you get:

• Gassen er farlig.
• Bilen er farlig.
• Huset er farlig.
• Barna er farlige. sometimes plural can change, but with singular after er, the base form is normal

The most important point for learners is this:

• before a noun, adjectives often change
• after er, they often stay in their basic form

Yes. Then the adjective comes before the noun.

• en farlig gass = a dangerous gas
• gassen er farlig = the gas is dangerous

So Norwegian can express the same idea in two different structures:

• adjective + noun
• noun + er + adjective

Gass is normally a masculine noun:

• en gass
• gassen

In Bokmål, many masculine nouns can sometimes be treated differently in certain styles, but en gass / gassen is the normal form to learn.

A simple learner-friendly pronunciation is roughly:

• gassen ≈ GAHS-sen

A few tips:

• The g is a normal hard g sound.
• The a is more like the a in father than the English a in cat.
• The ss is a clear s sound.
• The ending -en is usually pronounced lightly, something like -en or -n depending on speech style.

If you are learning standard Eastern Norwegian pronunciation, that is a good starting point.

No, this is the basic Norwegian word order for a simple statement:

subject + verb + complement

So:

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

This is very similar to English:

• The gas
  • is
    • dangerous

Yes, if the context already makes it clear that den refers to gassen.

• Gassen er farlig. = The gas is dangerous.
• Den er farlig. = It is dangerous.

Because gass is a common-gender noun, the pronoun is den, not det.

Farlig is the basic form. Farlige is often used in plural or in some definite noun phrases.

Examples:

• en farlig gass = a dangerous gas
• farlige gasser = dangerous gases
• de farlige gassene = the dangerous gases

But in your sentence, the adjective comes after er, so farlig is correct:

• Gassen er farlig.

Not usually. Gassen with the definite ending most naturally means the gas.

If you want to speak more generally, Norwegian often uses:

• Gass er farlig. = Gas is dangerous. or
• Gasser kan være farlige. = Gases can be dangerous.

So Gassen er farlig usually refers to a specific gas already known in the situation.

This sentence is standard Bokmål.

In Nynorsk, you would more naturally see:

• Gassen er farleg.

The main difference here is the adjective:

• Bokmål: farlig
• Nynorsk: farleg

No, not in standard Norwegian. The correct form is farlig.

English speakers sometimes expect an extra ending because adjectives can change in other Scandinavian languages, but in Norwegian:

• Gassen er farlig. is correct.

You may see farligt in Danish or in older/historical contexts, but not in normal modern Norwegian Bokmål.