Við stöðvum bílinn þegar umferðarljósið verður rautt.

A word-by-word breakdown is:

• Við = we
• stöðvum = stop
  
  • from the verb stöðva = to stop
• bílinn = the car
  • from bíll = car
• þegar = when
• umferðarljósið = the traffic light
  • literally a compound: umferð = traffic, ljós = light
• verður = becomes / turns
  • from verða = to become
• rautt = red
  • from rauður = red

So the whole sentence means something like We stop the car when the traffic light turns red.

Because stöðvum is the form of stöðva used with við = we.

The verb stöðva is an -a verb, and in the present tense:

• ég stöðva = I stop
• þú stöðvar = you stop
• hann/hún/það stöðvar = he/she/it stops
• við stöðvum = we stop
• þið stöðvið = you (plural) stop
• þeir/þær/þau stöðva = they stop

So við stöðvum means we stop.

Because bílinn is in the accusative case, not the nominative.

The noun bíll means car.
Its definite singular forms include:

• bíllinn = the car as subject, nominative
• bílinn = the car as direct object, accusative

In this sentence, the car is the thing being stopped, so it is the direct object of stöðvum. That is why Icelandic uses bílinn.

So:

• Bíllinn stoppar. = The car stops.
  Here bíllinn is the subject.
• Við stöðvum bílinn. = We stop the car.
  Here bílinn is the object.

Because Icelandic very often forms compound nouns by joining words together.

Here:

• umferð = traffic
• ljós = light
• umferðarljós = traffic light
• umferðarljósið = the traffic light

This is very normal in Icelandic. English often writes similar ideas as separate words, but Icelandic frequently combines them into one noun.

That ending is the definite article attached to the noun.

In Icelandic, the is usually not a separate word. Instead, it is added to the end of the noun.

So:

• umferðarljós = traffic light
• umferðarljósið = the traffic light

This noun is neuter singular, and -ið is the definite ending here.

Because verða means to become or to turn, while vera means to be.

So:

• umferðarljósið er rautt = the traffic light is red
• umferðarljósið verður rautt = the traffic light becomes / turns red

In this sentence, the light is changing from another color to red, so verður rautt is the natural choice.

Because the adjective must agree with umferðarljósið, which is:

• neuter
• singular
• here used as a predicate adjective after verður

The adjective rauður = red has different forms depending on gender:

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

Since umferðarljós is a neuter noun, Icelandic uses rautt.

Compare:

• bíllinn er rauður = the car is red
  (bíll is masculine)
• bókin er rauð = the book is red
  (bók is feminine)
• ljósið er rautt = the light is red
  (ljós is neuter)

Here þegar means when.

It introduces a time clause:

• þegar umferðarljósið verður rautt = when the traffic light turns red

A useful thing to know is that þegar can also mean already in other contexts. But in this sentence it is definitely the conjunction when.

Grammatically, the verbs are in the present tense, but the sentence can be understood in a broader way.

Við stöðvum bílinn þegar umferðarljósið verður rautt can mean:

• a general/habitual truth: We stop the car when the traffic light turns red
• or a future situation, depending on context: We will stop the car when the traffic light turns red

This is normal. Icelandic, like English, often uses present-tense forms in when-clauses for future meaning.

Compare English:

• We stop the car when the light turns red.
• We will stop the car when the light turns red.

Icelandic works similarly here.

Usually Icelandic does not drop subject pronouns the way some languages do.

So við stöðvum is the normal way to say we stop.

Because the verb ending -um already suggests we, the pronoun can sometimes feel a bit redundant from an English-speaking learner’s perspective, but Icelandic still normally uses it.

So in ordinary speech and writing:

• Við stöðvum bílinn is the standard form.

It is in the nominative singular.

That is because umferðarljósið is the subject of the clause þegar umferðarljósið verður rautt.

You can think of that clause as:

• the traffic light = subject
• turns = verb
• red = predicate adjective

So:

• umferðarljósið = nominative subject
• rautt agrees with it as a neuter singular adjective

Yes. You can put the þegar clause first:

• Þegar umferðarljósið verður rautt, stöðvum við bílinn.

This means the same thing: When the traffic light turns red, we stop the car.

Notice something important: in the main clause after the introductory clause, the verb comes before the subject:

• stöðvum við
• not við stöðvum

That is because Icelandic follows a V2 pattern in main clauses: the finite verb usually comes in the second position.

So both are correct:

• Við stöðvum bílinn þegar umferðarljósið verður rautt.
• Þegar umferðarljósið verður rautt, stöðvum við bílinn.

Yes, stöðva is being used transitively here, meaning it takes an object:

• stöðva bílinn = stop the car

That is a very natural choice if you want to say that we bring the car to a stop.

You may also hear stoppa in modern Icelandic, especially in speech, but stöðva is a very standard and good verb to learn.

A useful distinction is:

• Bíllinn stoppar. = The car stops.
  The car itself stops.
• Við stöðvum bílinn. = We stop the car.
  We cause the car to stop.

A rough pronunciation guide for English speakers is:

• Við ≈ vith
  (with ð like the th in this)
• stöðvum ≈ sturth-vum or stoe-thvum
• bílinn ≈ BEE-lin
• þegar ≈ THYE-gar
  (with þ like the th in thin)
• umferðarljósið ≈ UM-fer-thar-lyoh-see-ith
• verður ≈ VER-thur
• rautt ≈ royht or routt

A few key letter tips:

• þ = unvoiced th, as in thin
• ð = voiced th, as in this
• au is often pronounced somewhat like öy / oi
• ll and some other consonant combinations may sound different from what English speakers expect

This is only an approximation, but it can help you get started.

The sentence has two parts:

1. Við stöðvum bílinn
   = main clause
   we stop the car

2. þegar umferðarljósið verður rautt
   = subordinate time clause
   when the traffic light turns red

So the structure is:

• Subject + verb + object + þegar + subject + verb + predicate adjective

More specifically:

• Við = subject
• stöðvum = finite verb
• bílinn = direct object
• þegar = conjunction
• umferðarljósið = subject of the subordinate clause
• verður = finite verb
• rautt = predicate adjective

This is a very useful sentence pattern to learn, because you can build many similar sentences with it.