Ég sit í farþegasætinu þegar hún keyrir.

Because sit is the 1st person singular present tense form of the verb að sitja (to sit).

• að sitja = to sit
• ég sit = I sit / I am sitting

Icelandic verbs change depending on the subject, so you usually cannot use the infinitive after the subject the way English learners might be tempted to.

It can mean either one, depending on context.

Icelandic does not usually make a strong grammatical distinction between:

• I sit
• I am sitting

So ég sit can describe:

• a general situation, or
• what is happening right now

Context tells you which meaning is intended.

Here, í means in, and because the sentence describes a location rather than movement, í takes the dative case.

So:

• í farþegasætinu = in the passenger seat
• farþegasætinu is dative singular definite

A very useful rule is:

• í + accusative for movement into something
• í + dative for being in something

Compare:

• Ég sit í farþegasætinu. = I am sitting in the passenger seat.
• Ég fer í farþegasætið. = I go into the passenger seat / I get into the passenger seat.

Because Icelandic usually adds the definite article (the) to the end of the noun instead of using a separate word.

So:

• farþegasæti = passenger seat
• farþegasætið = the passenger seat (nominative/accusative)
• farþegasætinu = the passenger seat (dative)

This is very common in Icelandic. English uses a separate word (the), but Icelandic often uses an ending.

It is a compound noun, which is extremely common in Icelandic.

It breaks down as:

• farþegi = passenger
• sæti = seat

So farþegasæti literally means passenger-seat.

This kind of noun-building is very normal in Icelandic, just as in English with words like bedroom or toothbrush.

Keyrir is the 3rd person singular present tense of að keyra (to drive).

So:

• að keyra = to drive
• hún keyrir = she drives / she is driving

Again, Icelandic present tense can often cover both the simple present and the progressive meaning, depending on context.

It can mean either one grammatically, but in this sentence it will often be understood as is driving because of the situation being described.

So þegar hún keyrir can mean:

• when she drives
• when she is driving

The exact nuance comes from context rather than a special verb form.

Þegar means when and introduces a time clause.

So:

• þegar hún keyrir = when she drives / when she is driving

It connects the two parts of the sentence:

• Ég sit í farþegasætinu
• þegar hún keyrir

In other words, it tells you when the first action happens.

Yes. That is a perfectly natural Icelandic sentence.

When the þegar-clause comes first, Icelandic usually shows verb-second word order in the main clause:

• Þegar hún keyrir, sit ég í farþegasætinu.

Notice that it is sit ég, not ég sit, after the opening clause. That is a very important Icelandic word order pattern.

So:

• Ég sit í farþegasætinu þegar hún keyrir.
• Þegar hún keyrir, sit ég í farþegasætinu.

Both are correct; the second one gives a bit more focus to when she drives.

Because hún is the nominative form, and here it is the subject of the verb keyrir.

In Icelandic, pronouns change by case, just like nouns do. Since she is doing the action of driving, the nominative is used:

• hún = she

If it were an object or part of a different structure, a different case form could appear.

It has some forms that learners usually need to memorize carefully, so it is not one of the simplest verbs.

For example:

• að sitja = to sit
• ég sit = I sit
• við sitjum = we sit

The vowel changes in some forms, so it is worth learning it as a verb pattern rather than trying to guess every form from the infinitive.

Þ is pronounced like the th in English thing, not like the th in this.

So:

• þegar begins with the unvoiced th sound

This letter is called thorn and is a special letter in Icelandic.

A helpful contrast:

• þ = like th in thin
• ð = like th in this (roughly)

With seats and enclosed positions, Icelandic often uses í where English may still say in or sometimes think more loosely of on.

Here, í farþegasætinu is the normal way to say in the passenger seat.

This is a good reminder that prepositions do not always match one-for-one between English and Icelandic. It is best to learn them as part of whole expressions:

• sitja í sæti = sit in a seat
• sitja í farþegasætinu = sit in the passenger seat

Yes — several useful patterns appear here:

1. Verb forms change with the subject

   • ég sit
   • hún keyrir

2. The definite article is often attached to the noun

   • farþegasætinu = the passenger seat

3. Prepositions can control case

   • í takes dative here because it describes location

4. Icelandic present tense is flexible

   • sit = sit / am sitting
   • keyrir = drives / is driving

5. Word order changes after an opening clause

   • Þegar hún keyrir, sit ég ...

That makes this a very useful sentence for learning several core Icelandic patterns at once.