Learn Hardcore Icelandic: Foreldrarnir eru ekki lengur ungir, en þau leika sér enn með börnunum. - The parents are no longer young, but they still play with the children.

Here foreldrarnir refers to a pair of parents (typically one mother and one father, or in any case not all the same gender), so the neutral, mixed‑gender pronoun þau is natural.

You will also see sentences like:

Mamma og pabbi, þau búa í Reykjavík.
Mum and Dad, they live in Reykjavík.

So þau here just means they, referring back to foreldrarnir.

What exactly does leika sér mean? Why is sér there?

Leika sér is an idiomatic reflexive verb meaning to play in the sense of to amuse oneself, to play for fun.

leika on its own often means to play (an instrument, a role, a game), or to act in a play or film.
- Hún leikur í myndinni. – She acts in the movie.
- Hann leikur á píanó. – He plays the piano.
leika sér adds the reflexive pronoun sér (to oneself), and the whole expression means to play (for fun), usually children (or playful adults or animals):
- Krakkarnir leika sér úti. – The kids are playing outside.

In your sentence, þau leika sér enn með börnunum = they still play with the children (for fun). You cannot omit sér here; leika alone would change the meaning.

Why is it með börnunum and not just með börnin or something else?

The preposition með (with) here takes the dative case when it means together with someone.

The noun barn (child) is irregular:

Singular:
- Nom/Acc: barn
- Dat: barni
Plural:
- Nom/Acc: börn
- Dat: börnum

Then you add the definite article to the dative plural:

Dative plural indefinite: börnum
Dative plural definite: börnunum = the children

So:

með börnum = with children
með börnunum = with the children

Because the English sentence says with the children, Icelandic uses the definite form börnunum.

Why is the adjective ungir and not ung or ungt?

Adjectives in Icelandic agree in gender, number, and case with the noun they describe.

The subject here is foreldrarnir (the parents), which is grammatically masculine plural nominative.
The base adjective is ungur (young).

Masculine plural nominative form of ungur is ungir.

So:

foreldrarnir eru ungir = the parents are young

If the noun were feminine plural, you would see ungar; if neuter plural, ung.

What is the function of ekki lengur? Why not just ekki?

ekki = not
lengur (in negatives) = any longer, anymore

Together, ekki lengur means no longer / not anymore.

So:

Foreldrarnir eru ekki ungir. = The parents are not young.
Foreldrarnir eru ekki lengur ungir. = The parents are no longer young / The parents are not young anymore.

The word lengur adds the idea of a change over time: they used to be young, but not anymore.

Why is it lengur and not lengi?

Both lengi and lengur relate to a long time, but they are used differently:

lengi – for a long time (usually positive statements or questions)
- Hann bjó lengi í Reykjavík. – He lived in Reykjavík for a long time.
lengur – any longer, anymore (commonly in negative sentences and questions)
- Hann býr ekki lengur í Reykjavík. – He no longer lives in Reykjavík.

In eru ekki lengur ungir, the sentence is negative (not) and expresses the idea of any longer, so lengur is the correct choice.

Why is there both ekki lengur and enn in the same sentence? Aren’t they the same idea?

They express two different time contrasts in the two clauses:

Foreldrarnir eru ekki lengur ungir
– The parents are no longer young (they used to be, but time has passed).
en þau leika sér enn með börnunum
– but they still play with the children (despite not being young, this activity continues).

So:

ekki lengur = no longer / not anymore (something has stopped)
enn = still / yet (something continues)

The sentence is contrasting what has changed (their age) with what has not changed (they still play).

Could I say ennþá instead of enn in þau leika sér enn með börnunum?

Yes, you can. Both are natural:

þau leika sér enn með börnunum
þau leika sér ennþá með börnunum

ennþá is often a bit more colloquial and emphatic; enn is slightly shorter and more neutral. In meaning here, they are practically the same: still.

Why is there a comma before en?

en means but and introduces a new clause that contrasts with the first one:

Clause 1: Foreldrarnir eru ekki lengur ungir
Clause 2: en þau leika sér enn með börnunum

In Icelandic, as in English, you normally put a comma before a coordinating conjunction like en when it links two independent clauses. Compare English:

The parents are no longer young, but they still play with the children.

Could you omit þau and just say …, en leika sér enn með börnunum?

No. Icelandic is not a pro‑drop language like Spanish or Italian; you normally must state the subject pronoun.

So þau is required:

…, en þau leika sér enn með börnunum. – correct
…, en leika sér enn með börnunum. – ungrammatical (missing subject)

The pronoun þau clearly shows who is doing the playing (the parents).

Where can the adverb enn go? Could I say þau enn leika sér með börnunum?

In neutral word order, enn usually comes after the verb phrase starts, often after the verb or verb+reflexive:

þau leika sér enn með börnunum – natural
þau leika sér með börnunum enn – possible but sounds more marked/emphatic or poetic
þau enn leika sér með börnunum – sounds very unusual/wrong in normal modern prose

So the given word order (leika sér enn) is the standard, natural one.