Learn Hardcore Icelandic: Sumir vinir treysta bara fjölskyldunni sinni.

It is an adjective/pronoun that must agree with the noun it describes.
Vinir = friends is masculine plural nominative, so sumir is also masculine plural nominative.

So sumir vinir literally means “some friends” / “certain friends”, and it functions as the subject of the sentence: Some friends (sumir vinir) trust only their family.

Why is vinir in that form and not something like vini or vina?

Vinir is the nominative plural of vinur (“friend”):

singular: vinur (friend) – nominative
plural: vinir (friends) – nominative

We use the nominative case for the subject of the sentence.
Since friends are doing the action (trust), vinir must be in nominative plural: sumir vinir = “some friends”.

What exactly does treysta mean here? Is it “trust” or “rely on”?

In this sentence, treysta means “to trust”.

treysta e-m (with a person in the dative) = “to trust someone”.
treysta á e-ð = “to rely on something/someone, depend on”.

Here we have treysta followed directly by fjölskyldunni sinni (in the dative), so the meaning is “trust (their own family)”, not specifically “rely on” in the depend on sense, although in practice trust and rely on often overlap.

Why is fjölskyldunni in that particular form? What case is it?

Fjölskyldunni is dative singular definite of fjölskylda (“family”).

Basic noun: fjölskylda – “family” (feminine)
Nominative singular definite: fjölskyldan – “the family”
Dative singular indefinite: fjölskyldu
Dative singular definite: fjölskyldunni

The verb treysta always takes its object in the dative case:

treysta e-m = “to trust someone” (someone = dative)

So we must say fjölskyldunni (dative) after treysta, and because we mean their own family (a specific one), we use the definite form: the family → fjölskyldunni.

What is sinni exactly, and how is it different from a normal “their”?

Sinni is a form of the reflexive possessive pronoun sinn. It means “his/her/its/their own”, referring back to the subject of the sentence.

In this sentence:

Subject: sumir vinir (“some friends”)
Possessed noun: fjölskyldunni (“the family”, fem. dat. sg.)
Reflexive possessive agreeing with that noun: sinni (feminine, dative, singular)

So fjölskyldunni sinni literally is “the family [their own]”.

Difference from ordinary “their”:

fjölskyldu þeirra = “their family” (could be someone else’s family, not necessarily the subject’s own)
fjölskyldu sinni = “their own family” (must belong to the subject of the same clause)

Here, sinni tells you the family belongs to those same friends who are doing the trusting.

Why is it fjölskyldunni sinni and not sinni fjölskyldunni, like English “their family”?

In Icelandic, possessive pronouns usually come after the noun, especially in neutral, unmarked word order:

fjölskyldan mín – “my family”
vinir okkar – “our friends”
fjölskyldunni sinni – “their own family”

Putting the possessive before the noun is possible but usually more marked or emphatic, and with sinn it’s less common in simple sentences.

So fjölskyldunni sinni is the normal, natural order: family (theirs) rather than their family.

Why is sinni in that form and not sínum or sinnar?

The reflexive pronoun sinn must agree with the gender, number, and case of the noun it modifies:

Noun: fjölskyldunni
- gender: feminine
- number: singular
- case: dative

So sinn takes the form:

feminine, dative, singular → sinni

Other forms you might see:

sínum – dative masculine or neuter singular, or dative plural
sinnar – genitive feminine singular or nominative/genitive feminine plural

Here only sinni matches fjölskyldunni, so that is the correct form.

Why is bara in the middle of the sentence, and what exactly does it mean here?

Bara is an adverb meaning “only, just” (also “simply” or “merely” in other contexts).

In Sumir vinir treysta bara fjölskyldunni sinni, bara is placed right after the verb treysta to show that it limits what they trust:

treysta bara fjölskyldunni sinni
→ they only trust their family (and not other people).

This placement gives bara scope over the object (“only [X]”), not over the subject. Compare:

Bara sumir vinir treysta fjölskyldunni sinni
→ only some friends trust their family (others don’t).

So where you put bara changes what is being limited.

Could I say aðeins instead of bara? Is there a difference?

You can say aðeins in many places where you say bara, and it also often means “only”:

Sumir vinir treysta aðeins fjölskyldunni sinni.

However, there are nuances:

bara is very common and informal; it can mean “only / just / simply / merely”.
aðeins feels a bit more neutral or formal in many contexts, and less “colloquial-sounding”.

In this particular sentence, both are grammatically fine. Native speakers probably say bara more often in everyday speech.

Why do we use sinni instead of þeirra if we’re talking about “their family”?

Because the family belongs to the same people as the subject (“some friends”), we must use the reflexive pronoun sinn:

Sumir vinir treysta bara fjölskyldunni sinni.
→ Some friends only trust their own family.

If you used þeirra instead, the meaning changes:

Sumir vinir treysta bara fjölskyldu þeirra.
→ Some friends only trust their family (someone else’s, not necessarily their own).

So:

sinni → refers back to the subject (their own).
þeirra → a normal third‑person possessive, can refer to some other group.

Could I drop the definite ending and say fjölskyldu sinni instead of fjölskyldunni sinni?

You can say:

Sumir vinir treysta bara fjölskyldu sinni.

Grammatically it’s fine: fjölskyldu (dative fem. sg. without the definite article) + sinni.

The difference:

fjölskyldunni sinni (definite) feels more like “their (own) family” as a clearly identified unit.
fjölskyldu sinni (indefinite) is closer to “a family of theirs”, though in many real contexts the meaning will still be understood as “their own family”.

In everyday speech, with this kind of meaning, the definite form (fjölskyldunni sinni) is very natural because we usually think of the family, not just a family.

Why is the verb treysta in the form treysta and not something like treysta with another ending?

Here treysta is the 3rd person plural present tense:

infinitive: að treysta – to trust
1st sg.: ég treysti
2nd sg.: þú treystir
3rd sg.: hann/hún/það treystir
1st pl.: við treystum
2nd pl.: þið treystið
3rd pl.: þeir/þær/þau treysta

Since the subject is sumir vinir (“some friends” → they), we need 3rd person plural, which is treysta.