Barnið situr á jörðinni.

Because barnið is the noun barn (child) with the definite article attached to the end.

• barn = child
• barnið = the child

This is very common in Icelandic: instead of a separate word like English the, Icelandic often adds the article onto the noun itself.

Also, barn is a neuter noun, and -ið is the definite ending here.

In Icelandic, grammatical gender and real-life sex/gender are not always the same thing.

The word barn is simply a neuter noun by grammar, no matter whether the child is a boy or a girl. So you say:

• barn = child
• barnið = the child

This is something learners just have to memorize with each noun: masculine, feminine, or neuter.

Situr is the present tense, 3rd person singular form of the verb sitja (to sit).

So:

• ég sit = I sit / I am sitting
• þú situr = you sit / you are sitting
• hann/hún/það situr = he/she/it sits / is sitting

Since barnið is singular, the verb is also singular: situr.

In Icelandic, the simple present often covers what English expresses as both:

• sits
• is sitting

So Barnið situr á jörðinni can mean The child sits on the ground or more naturally The child is sitting on the ground, depending on context.

Because á is a preposition, and here it means on in the sense of location: the child is already sitting there.

With many Icelandic prepositions, the case depends on whether there is:

• location / no movement → often dative
• motion toward a place → often accusative

Here there is no movement onto the ground; it is a static situation, so á takes the dative.

The noun is jörð (earth, ground, feminine), and in this sentence it appears as:

• jörðinni = the ground in the dative singular definite

So:

• á jörðinni = on the ground

No. Á can take different cases depending on meaning.

A very important pattern is:

• á + dative for location: on
• á + accusative for movement onto something

Here:

• Barnið situr á jörðinni. = The child is sitting on the ground.
  Static location, so dative.

Compare that idea with a movement sentence such as:

• Barnið leggst á jörðina. = The child lies down onto the ground.
  Movement toward the surface, so accusative.

So the case after á helps show whether something is already somewhere or moving there.

In this sentence, jörðinni most naturally means the ground.

The noun jörð can mean different things depending on context, such as:

• earth as the planet or the world
• soil
• ground
• sometimes land

But in Barnið situr á jörðinni, the natural English meaning is on the ground.

For the same reason as in barnið: Icelandic often uses a suffixed definite article.

So:

• jörð = ground / earth
• jörðin = the ground / the earth (nominative)
• jörðinni = the ground in the dative

Because á here requires the dative, the definite form is jörðinni, not jörðin.

So the sentence literally breaks down like this:

• Barnið = the child
• situr = sits / is sitting
• á jörðinni = on the ground

The basic order here is:

• Subject + Verb + Prepositional phrase

So:

• Barnið = subject
• situr = verb
• á jörðinni = where the action happens

That is a very normal Icelandic sentence pattern.

However, Icelandic also has verb-second word order in main clauses, so other word orders are possible for emphasis. For example, if another element comes first, the verb usually still stays in second position.

But Barnið situr á jörðinni is the straightforward neutral version.

Technically yes, because jörð can mean earth as well as ground. But in normal everyday context, this sentence is understood as:

• The child is sitting on the ground

That is the natural translation most learners should take from it unless the context clearly points to something more unusual.

A rough learner-friendly approximation is:

BAR-nith SI-tur au YERTH-inni

A few notes:

• ð in barnið and jörðinni is like the th in this, though pronunciation can vary depending on position.
• j in Icelandic is pronounced like English y, so jörð starts roughly like yer-.
• á is pronounced like ow in now, but longer.
• The rð cluster in jörðinni can be tricky and may not sound exactly like English sounds.

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

Yes: Icelandic usually does not need a separate form like English is sitting.

English often distinguishes:

• The child sits
• The child is sitting

Icelandic often just uses the simple present:

• Barnið situr

Context tells you whether it means a general fact or something happening right now. In this sentence, the most natural English translation is usually:

• The child is sitting on the ground

So one useful habit for learners is: do not expect Icelandic to match English progressive forms word for word.