Learn Hardcore Icelandic: Hjónin ræða oft um aldur barna sinna. - The couple often talk about the age of their children.

hjón means “a married couple” (two people, treated as a pair).
Grammatically, hjón is neuter plural in form, even though it refers to exactly one couple.
hjónin is hjón with the definite article, so it means “the married couple”.

So even though you’re talking about one couple, the word itself is plural, and it always takes plural verb agreement (like ræða here). This is why you see Hjónin ræða… and not Hjónin ræðir…

Why is the verb ræða in the form ræða and not ræðir or something else?

The infinitive of the verb is að ræða = “to discuss / talk about”.

Present tense forms (active, indicative) are:

1st person singular: ég ræði – I discuss
2nd person singular: þú ræðir – you discuss
3rd person singular: hann / hún / það ræðir – he / she / it discusses
Plural (all persons): við / þið / þeir / þær / þau ræða – we / you (pl.) / they discuss

Because hjónin is grammatically plural, the verb must also be plural ræða:

Hjónin ræða oft … = The (married) couple often discuss …

If the subject were singular, you’d use ræðir:

Maðurinn ræðir oft … = The man often discusses …

Why is oft placed between ræða and um aldur? Can oft go somewhere else in the sentence?

oft is an adverb meaning “often”. In Icelandic, adverbs of frequency like oft most commonly appear:

After the finite verb and before the main object/prepositional phrase.

So:

Hjónin ræða oft um aldur barna sinna.

This is the most natural word order.

Other positions are possible, but change the focus or sound less neutral:

Hjónin ræða um aldur barna sinna oft. – possible, but sounds a bit marked or as if you’re emphasizing the time/frequency after everything else.
Oft ræða hjónin um aldur barna sinna. – fronting oft for emphasis: “Often, the couple talk about the age of their children.”

The version in your sentence is the default, neutral word order.

Why do we need the preposition um here? Could we just say Hjónin ræða aldur barna sinna?

The verb ræða is very often used together with the preposition um when it means “talk about / discuss (a topic):

ræða um eitthvað = discuss something / talk about something

Without um, ræða can have a more formal or direct-object meaning like “to negotiate / to deal with / to discuss (as a direct object)”, but for everyday “talk about X”, um is normal and natural.

You can say ræða aldur barna sinna, and it is grammatically possible, but it sounds more formal or slightly more “topic as direct object”. Native usage in everyday speech here strongly prefers:

ræða um aldur barna sinna = “talk about the age of their children”

What case is aldur in, and why?

aldur (age) is a masculine noun. Its base form (nominative singular) is aldur.

In this sentence, it appears after the preposition um, and um takes the accusative case. So:

um + aldur → aldur in the accusative singular.

The form happens to be identical to the nominative in this noun, but grammatically it is accusative here because um always requires the accusative:

um + ACC:
- um aldur – about (the) age
- um bílinn – about the car
- um bæinn – about the town

Why is it barna instead of börn? What form is barna?

The noun barn (child) declines like this:

Nominative singular: barn – child
Nominative/accusative plural: börn – children
Genitive plural: barna – of (the) children

In aldur barna sinna, we have a possessive/genitive construction:

aldur barna = “the age of (the) children”

So barna is genitive plural, which is required because we are expressing “the age of their children” (just like English uses “of” or ’s).

If you used börn instead:

aldur börn sinna – grammatically wrong, because börn here would be the wrong case (nominative/accusative instead of genitive).

What exactly is sinna, and how is it different from þeirra for “their”?

sinna here is the genitive plural of the reflexive possessive pronoun sinn / sín / sitt.

sinn / sín / sitt means “his/her/their own” and always refers back to the subject of the sentence.
þeirra is the ordinary genitive plural of þeir (“they”) and is a non‑reflexive possessive: it can refer to some other group, not necessarily the subject.

In your sentence:

Hjónin ræða oft um aldur barna sinna.
→ sinna tells us that the children belong to the same “they” as the subject, i.e. the couple’s own children.

If you used þeirra:

Hjónin ræða oft um aldur barna þeirra.

This can easily be understood as: The couple often talk about the age of *other people’s children* (some third party).

So:

barna sinna = the age of their own children (the couple’s children)
barna þeirra = the age of their children, possibly belonging to some other “they”

Why is sinna placed after barna? In English we say “their children,” not “children their.”

In Icelandic, possessive pronouns can go:

Before the noun (more emphatic or contrastive), or
After the noun (more neutral, very common in written style).

For plural children:

börn þeirra or börnin þeirra – their children / the children of theirs
barna sinna – of their (own) children

In your specific phrase aldur barna sinna:

aldur (nominative/accusative)
barna (genitive plural: “of children”)
sinna (genitive plural: “their own”)

So the structure literally is “age [of children their-own]”. That post‑position of the possessive is very natural in Icelandic, especially in genitive phrases.

Could we say Hjónin tala oft um aldur barna sinna instead? What is the difference between ræða and tala um?

Yes, you can say:

Hjónin tala oft um aldur barna sinna.

This is perfectly correct.

Difference in nuance:

ræða (um) – “discuss, talk about”
- Slightly more formal or suggests a more focused, maybe serious discussion.
tala um – “talk about”
- More general, common everyday verb for talking/speaking.

In many contexts, they overlap. Here, both sentences mean essentially the same thing; ræða um might sound just a touch more “discuss” than casual “chat”, but it’s a subtle difference.

What is the grammatical gender and number of hjón / hjónin?

The noun hjón has these key properties:

Gender: neuter
Number: plural form, even though it refers to one married couple
Meaning: (one) married couple (two people as a unit)

The definite form:

hjónin = “the married couple”

Because the word is grammatically plural:

It takes plural verb agreement: Hjónin ræða…
Pronouns referring back can be plural þau (they, neuter plural).

How would the sentence change if we talked about only one parent instead of a couple?

If the subject is singular, the verb must be singular, and we probably change the noun as well. For example:

Foreldrið ræðir oft um aldur barna sinna.
- foreldrið = “the parent” (singular)
- ræðir = 3rd person singular
- barna sinna still works: “of his/her own children”

More natural in plural “parents”:

Foreldrarnir ræða oft um aldur barna sinna.
- Foreldrarnir = “the parents” (plural)
- ræða = plural verb

The original with Hjónin specifically emphasizes that the two people are a married couple, not just parents in general.

How would this sentence look in the past tense?

Past tense of að ræða (3rd person plural) is ræddu.

So:

Hjónin ræða oft um aldur barna sinna. – The couple often talk/discuss… (present)
Hjónin ræddu oft um aldur barna sinna. – The couple often talked/discussed the age of their children. (past)

Everything else in the sentence stays the same; only the verb form changes.