Þau sem eru í sandkassanum fara oft á róluna líka.

Here þau is best understood as those ones / they, depending on context.

In this sentence, þau sem ... means those who ... or the ones who .... So þau is pointing to a group, and the clause after it identifies which group.

A native English speaker often expects they, but Icelandic very naturally uses a pronoun this way before sem:

• þau sem eru í sandkassanum = those who are in the sandbox

Also, þau is the neuter plural form. It is often used:

• for a mixed group of people
• for children
• for something previously referred to by a neuter plural noun

So in playground contexts, þau very often refers to the children.

Because Icelandic pronouns show grammatical gender.

Plural personal/demonstrative-style pronouns are:

• þeir = masculine plural
• þær = feminine plural
• þau = neuter plural

Þau is commonly used for:

• a mixed-gender group
• children as a group
• a group referred to by a neuter noun such as börn (children, which is grammatically neuter plural)

So even if English would just say they, Icelandic has to choose a gendered plural form, and þau is the natural one here.

Sem introduces a relative clause. In English, this is where we often use who, that, or which.

So:

• þau sem eru í sandkassanum = the ones who are in the sandbox

One useful thing for learners: sem does not change form. Unlike English, where you might choose between who, that, or which, Icelandic uses sem very broadly.

Because the subject is plural.

The verb vera (to be) changes with number:

• er = is
• eru = are

Since þau is plural, the sentence needs eru:

• þau eru = they are

So:

• þau sem eru í sandkassanum = those who are in the sandbox

Because this sentence describes location, not motion into something.

With í:

• dative is used for being in a place
• accusative is used for motion into a place

So:

• í sandkassanum = in the sandbox (location, dative)
• í sandkassann = into the sandbox (movement, accusative)

Here the children are already in the sandbox, so Icelandic uses the dative form sandkassanum.

It comes from the noun sandkassi (sandbox), which is a masculine noun.

The form sandkassanum is:

• singular
• definite = the sandbox
• dative

A rough breakdown is:

• sandkassi = sandbox
• sandkassanum / sandkassanum = in/to/from the sandbox, depending on the preposition and case function

What matters most for a learner is that Icelandic usually puts the on the end of the noun instead of using a separate word.

So:

• sandkassi = a sandbox
• sandkassinn = the sandbox
• í sandkassanum = in the sandbox

Because fara expresses movement, and with á, movement usually takes the accusative.

So:

• fara á róluna = go onto/to the swing
• vera á rólunni = be on the swing

This is a very important Icelandic pattern:

• á + accusative = movement onto/toward
• á + dative = location on

So the sentence is describing moving from the sandbox to the swing, which is why it uses róluna and not rólunni.

Róluna is the accusative singular definite form of róla (swing).

Basic forms:

• róla = a swing
• rólan = the swing
• róluna = the swing (accusative)
• rólunni = on/to/from the swing in dative contexts

In this sentence, á with motion requires the accusative, so we get á róluna.

Yes, it could be plural, but the singular is natural if the speaker is talking about the swing as a typical destination or one specific swing.

So á róluna can mean:

• onto that particular swing
• to the swing area / the swing as the next activity

If the speaker meant several swings, they could say:

• á rólurnar = onto the swings

Icelandic often uses the singular definite in situations where English might also use a singular generic expression.

Because Icelandic usually uses a suffixed definite article.

Instead of a separate word like English the, Icelandic often attaches the definite article to the noun:

• sandkassi = sandbox

• sandkassinn = the sandbox

• róla = swing
• rólan = the swing

When the noun changes case, the full form changes too:

• í sandkassanum
• á róluna

So the idea of the is still there; it is just built into the noun form.

Because the whole phrase Þau sem eru í sandkassanum is the subject, and Icelandic main clauses normally place the finite verb in second position.

The first element of the sentence is not just þau by itself, but the entire subject:

• Þau sem eru í sandkassanum = first sentence element
• fara = finite verb, in second position

This is a good example of Icelandic V2 word order. Even though there is another verb inside the relative clause (eru), that does not change the structure of the main clause.

• oft = often
• líka = also / too

So the sentence says that these children often go to the swing too.

Their positions are natural Icelandic adverb placement:

• fara oft á róluna = often go on/to the swing
• líka at the end adds also/too/as well

Sentence-final líka is very common and natural. English often puts too at the end as well, so this part is fairly easy to relate to:

• ... fara oft á róluna líka = ... often go on the swing too

Yes, that is a very natural translation.

More literally, Icelandic is expressing motion toward/onto the swing, but in normal English go on the swing is the best equivalent.

So while the grammar is:

• fara = go
• á róluna = onto/to the swing

the most natural English is usually:

• go on the swing

This is a good example of where you should learn the Icelandic structure, but not force the English to be overly literal.