Viðvörunarljósið er rautt.

Icelandic very often makes compound nouns, where English might use two or three separate words.

Viðvörunarljósið can be broken down like this:

• viðvörun = warning
• ljós = light
• -ið = the (attached to the noun)

So:

• viðvörunarljós = warning light
• viðvörunarljósið = the warning light

This is extremely normal in Icelandic. Long words are often just smaller meaningful parts joined together.

The dictionary form is viðvörunarljós.

That is the indefinite singular form, meaning a warning light.

The form in your sentence, viðvörunarljósið, is the definite singular form, meaning the warning light.

So if you look it up in a dictionary, you would usually search for viðvörunarljós, not viðvörunarljósið.

That is just how Icelandic normally expresses definiteness. Instead of a separate article like English the, Icelandic usually adds the definite article as an ending.

So:

• ljós = a light / light
• ljósið = the light

And with the compound noun:

• viðvörunarljós = warning light
• viðvörunarljósið = the warning light

This is one of the first big differences English speakers notice.

Viðvörunarljós is neuter.

Gender matters in Icelandic because adjectives and articles must agree with the noun. Since viðvörunarljós is neuter singular, the adjective also appears in a neuter singular form:

• rautt = red (neuter singular)

That is why the sentence says:

• Viðvörunarljósið er rautt

and not rauður or rauð.

Because Icelandic adjectives must agree with the noun in gender, number, and often case.

The noun here, viðvörunarljósið, is:

• singular
• neuter
• nominative

So the adjective must match that:

• masculine singular: rauður
• feminine singular: rauð
• neuter singular: rautt

Since viðvörunarljósið is neuter singular, rautt is the correct form.

That is the normal neuter singular form of the adjective rauður.

The adjective changes by gender:

• rauður = masculine
• rauð = feminine
• rautt = neuter

So the -tt ending is not random; it is part of the adjective’s inflection pattern.

Many common Icelandic adjectives change form like this, so it is worth learning them as a set rather than as isolated words.

Er is the present tense, third-person singular form of the verb vera, which means to be.

So:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is

In your sentence, viðvörunarljósið is a singular subject, so Icelandic uses er:

• Viðvörunarljósið er rautt = The warning light is red

It is in the nominative case, because it is the subject of the sentence.

In a basic sentence of the type X is Y, the thing being described is usually in the nominative:

• Viðvörunarljósið = subject
• er = is
• rautt = predicate adjective describing the subject

So this is a straightforward nominative subject sentence.

Because rautt is not directly attached to the noun inside a noun phrase here. It is a predicate adjective, used after to be.

Compare:

• rautt viðvörunarljós = a red warning light
• Viðvörunarljósið er rautt = the warning light is red

In the first example, the adjective comes before the noun because it is part of the noun phrase.

In your sentence, the structure is:

• subject + er
  • adjective

So the adjective comes after the verb, just like in English.

Yes, if the context already makes it clear which light you mean.

• Ljósið er rautt = The light is red
• Viðvörunarljósið er rautt = The warning light is red

The longer form is more specific. The shorter form is natural when everyone already knows you are talking about the warning light.

A rough learner-friendly approximation is:

VIHT-vor-unar-lyoh-sith er RÖYT

A few notes:

• ð in viðvörun is like a soft th sound in English this
• ljó is difficult for English speakers; the lj sound is special in Icelandic
• ó is like a long o-type sound
• au in rautt is pronounced somewhat like öy / oy
• tt in rautt is pronounced as a voiceless consonant cluster, not exactly like English tt

Pronunciation varies a little by speaker, but the main challenge here is the compound noun and the vowel in rautt.

The indefinite version would be:

Viðvörunarljós er rautt.

Grammatically, that means A warning light is red.

However, in real usage, that sentence may sound a little odd unless you are speaking very generally or in a special context. In many situations, the definite version is more natural, especially if you mean a specific warning light on a machine, dashboard, or control panel.

That would be:

Viðvörunarljósin eru rauð.

Here several things change:

• viðvörunarljósið → viðvörunarljósin
  singular definite → plural definite
• er → eru
  singular is → plural are
• rautt → rauð
  neuter singular → neuter plural

So this is a good example of agreement happening across the whole sentence.

Yes. It is a very basic and very common pattern:

[subject] + vera + [adjective]

For example:

• Bíllinn er stór. = The car is big.
• Húsið er gamalt. = The house is old.
• Viðvörunarljósið er rautt. = The warning light is red.

So your sentence is a great example of an everyday Icelandic structure.