Hún situr fyrst á rólunni og fer svo á rennibrautina.

Hún means she. It is the 3rd person singular feminine subject pronoun.

In this sentence, hún is the subject of both verbs:

• Hún situr ...
• ... og fer ...

Icelandic often keeps the subject at the beginning of the sentence, just as English does in a basic statement.

Sitja is the infinitive, meaning to sit.

Situr is the present tense form meaning sits or is sitting. Icelandic verbs change form depending on the subject and tense.

So:

• að sitja = to sit
• hún situr = she sits / she is sitting

The same thing happens with fer:

• að fara = to go
• hún fer = she goes

Fyrst means first, and svo means then / afterwards.

Together they create a very common sequence pattern:

• fyrst ... svo ... = first ... then ...

So the sentence is structured like this:

• Hún situr fyrst á rólunni = she first sits on the swing
• og fer svo á rennibrautina = and then goes to/onto the slide

This is a very natural way to describe actions in order.

Because á is a very common Icelandic preposition meaning things like on, onto, or sometimes to, depending on the context.

Here it is used with playground equipment:

• á rólunni = on the swing
• á rennibrautina = onto/to the slide

The important point is that Icelandic prepositions often do not match English one-for-one. English might say:

• on the swing
• on the slide
• down the slide
• to the slide

But Icelandic may still use á in several of those situations.

This is a classic Icelandic pattern: the case changes depending on whether you are talking about location or movement toward a place.

With á:

• dative often expresses location
• accusative often expresses motion toward / onto

So:

• á rólunni = on the swing
  She is sitting there, so this is location → dative

• á rennibrautina = onto/to the slide
  She is going toward it, so this is movement → accusative

This location-vs-movement contrast is very important in Icelandic.

The dictionary forms are:

• róla = swing
• rennibraut = slide

In the sentence, both nouns are definite, meaning the swing and the slide.

Their forms here are:

• rólunni = the swing in the dative singular
• rennibrautina = the slide in the accusative singular

So the endings are showing both:

1. definiteness (the), and
2. case.

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

So:

• róla = a swing / swing
• rólan = the swing
  But after a preposition requiring dative, it becomes:
• rólunni = on the swing

And:

• rennibraut = a slide / slide
• rennibrautin = the slide
• rennibrautina = the slide in the accusative

So Icelandic often builds the into the noun itself.

Yes, fer is from að fara, which usually means to go, to travel, or to move somewhere.

In this sentence, fer svo á rennibrautina means something like:

• then goes to the slide
• then goes onto the slide

In context, it probably implies that she moves from the swing to the slide as the next activity.

So fer is not about a very specific kind of movement; it is a general verb of going.

Yes, Icelandic word order is somewhat flexible, but the version you have is very natural and neutral.

This sentence follows a straightforward pattern:

• Hún = subject
• situr = verb
• fyrst = time adverb
• á rólunni = prepositional phrase
• og fer svo á rennibrautina = second clause

You may also see adverbs moved around in Icelandic, but learners should treat this version as a very good standard model:

• Hún situr fyrst á rólunni og fer svo á rennibrautina.

Because og simply links the two actions in one sentence:

• Hún situr fyrst á rólunni
• og fer svo á rennibrautina

This works much like English:

• She first sits on the swing and then goes to the slide.

The subject hún does not need to be repeated in the second part because it is understood to be the same person.

It can cover both ideas, depending on context.

Icelandic present tense often corresponds to either:

• simple present: she sits
• present progressive: she is sitting

So hún situr á rólunni could mean:

• she sits on the swing
• she is sitting on the swing

Usually the surrounding context tells you which English translation sounds best.