Hún sveik vinkonu sína.

Because hún is the nominative form, and the subject of a finite clause in Icelandic is normally in the nominative.

Here, hún is the person doing the action, so nominative is required.

• hún = she as subject
• hana = her as object

So:

• Hún sveik vinkonu sína. = she did it
• If her were the object, you would use hana

Because vinkonu is the accusative singular form of vinkona.

The verb svíkja takes a direct object, and direct objects are very often in the accusative in Icelandic. So vinkona changes form:

• vinkona = nominative singular
• vinkonu = accusative singular

So the sentence uses vinkonu because friend is the object of betrayed.

Sveik is the past tense singular form of the verb svíkja.

A few useful forms are:

• infinitive: svíkja
• present, 3rd person singular: svíkur
• past, singular: sveik
• past, plural: sviku

So Hún sveik ... means she betrayed ... in the past.

This is a strong verb, so the vowel changes instead of just adding a regular ending.

Sína is the reflexive possessive form of sinn.

It means something like her own, though in natural English that own is often not stated.

So in this sentence, sína shows that the friend belongs with, or is associated with, the subject hún.

In other words, it means she betrayed her own friend, not someone else’s friend.

Because Icelandic normally uses sinn / sín / sitt when the possessor is the subject of the same clause.

Here, the subject is hún, and the friend is her own friend, so Icelandic uses sína.

• vinkonu sína = her own friend
• vinkonu hennar = her friend, but usually meaning another woman’s friend, not the subject’s own

So this distinction is much clearer in Icelandic than in English.

It agrees with the thing possessed, not with the owner.

That is an important point for English speakers.

Here, sína agrees with vinkonu, which is:

• feminine
• singular
• accusative

So the form is sína.

Even if the subject changed, the form could stay the same as long as the possessed noun stayed feminine singular accusative:

• Hún sveik vinkonu sína.
• Hann sveik vinkonu sína.

In both sentences, sína is the same because it agrees with vinkonu, not with hún/hann.

Because in Icelandic, possessives very often come after the noun in neutral, everyday wording.

So vinkonu sína is the normal order here.

English usually puts possessives before the noun: her friend. Icelandic often prefers:

• noun + possessive

For that reason, vinkonu sína sounds natural and standard.

A different order can sometimes be used for emphasis or style, but the post-noun position is the default one learners should expect most often.

Yes. Vinkona is specifically a female friend.

That is why the noun itself is feminine. If you wanted a male friend, you would normally use vinur.

So the sentence is specifically about betraying a female friend, not just any friend in a gender-neutral sense.

This is something Icelandic often marks more clearly than English does.