Bílstjórinn stöðvar bílinn við gangbrautina af því að barnið er að fara yfir götuna.

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word.

For example:

• bílstjórinn = the driver
• bílinn = the car
• barnið = the child
• gangbrautina = the pedestrian crossing
• götuna = the street

So instead of a separate word like English the, Icelandic often uses an ending such as -inn, -ið, or -na, depending on gender, number, and case.

Because bílinn is the accusative form, not the nominative form.

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, bílinn is the thing being stopped, so it is the direct object of stöðvar. That is why accusative is used.

Stöðvar is the 3rd person singular present tense of stöðva, which means to stop something.

So:

• bílstjórinn stöðvar bílinn = the driver stops the car

This is a transitive verb here: someone stops something.

That is slightly different from English stop, which can be either transitive or intransitive. In Icelandic, this sentence focuses on the driver causing the car to stop.

In this sentence, við means by, at, or near.

So:

• við gangbrautina = at/by the pedestrian crossing

A very common beginner mistake is to assume við always means with, but the usual Icelandic word for with is með.

Við has several uses, and one common one is location near something.

Because við takes the accusative case.

So the noun gangbraut appears here as:

• gangbrautin = nominative definite
• gangbrautina = accusative definite

Since við governs accusative, you get við gangbrautina.

This is something you often just have to learn together with the preposition:
við + accusative.

Because yfir here means across, and with movement across something it takes the accusative.

So:

• gata = street
• gatan = the street, nominative
• götuna = the street, accusative

In fara yfir götuna, the child is moving across the street, so accusative is used.

A useful pattern to remember is:

• yfir + accusative for motion across something

Af því að means because.

It is best learned as a fixed expression:

• af því að = because

You do not need to worry too much about the literal meaning of each individual word at first. As a learner, it is most useful to treat it as one chunk that introduces a reason clause.

So the sentence structure is:

• main clause: Bílstjórinn stöðvar bílinn við gangbrautina
• reason clause: af því að barnið er að fara yfir götuna

Er að + infinitive is a very common Icelandic way to express an action that is in progress or about to happen.

So:

• barnið er að fara yfir götuna = the child is crossing the street or is going across the street

It often corresponds to the English progressive form, like is going, is doing, is crossing.

Compare:

• barnið fer yfir götuna = more neutral, simple present
• barnið er að fara yfir götuna = the child is in the process of doing it, or it is happening right now

In this sentence, it makes sense as is crossing.

Because af því að introduces a subordinate clause, and subordinate clauses do not follow the usual main-clause verb-second pattern in the same way.

In a main clause, Icelandic often puts the finite verb in second position:

• Bílstjórinn stöðvar bílinn...

But after a subordinating conjunction like af því að, the clause normally has:

• conjunction + subject + finite verb

So:

• af því að barnið er að fara yfir götuna

That is normal Icelandic word order for a subordinate clause.

Because Icelandic nouns change form for:

• gender
• case
• number
• definiteness

That means the ending depends on both the noun itself and its role in the sentence.

Here are the forms in this sentence:

• bílstjórinn: masculine, singular, nominative, definite
• bílinn: masculine, singular, accusative, definite
• barnið: neuter, singular, nominative, definite
• gangbrautina: feminine, singular, accusative, definite
• götuna: feminine, singular, accusative, definite

So the different endings are not random. They show grammatical information that English usually does not show on the noun itself.