Hún elskar hundinn sinn.

In Icelandic, the definite article the is usually attached to the end of the noun instead of standing as a separate word.

So:

• hundur = dog
• hundinn = the dog

The ending -inn here is the definite article attached to the noun.

Because hundinn is the form needed in this sentence.

The verb elskar (loves) takes a direct object, and that object is normally in the accusative case. The noun hundur is the dictionary form, which is nominative singular. But here the dog is the object, so Icelandic uses the accusative singular form:

• hundur = nominative, dog
• hundinn = accusative definite, the dog

So the sentence uses hundinn because the dog is what she loves.

The basic form is elska, meaning to love.

Elskar is the present-tense form used with hún (she):

• ég elska = I love
• þú elskar = you love
• hún elskar = she loves

So elskar means loves here.

Because sinn is a reflexive possessive. It is used when the possessor is the subject of the same clause.

In Hún elskar hundinn sinn, the subject is hún (she), and the dog belongs to that same person. So sinn means her own.

This is different from hennar, which would normally mean her referring to some other female person, not the subject.

So:

• Hún elskar hundinn sinn = She loves her own dog.
• Hún elskar hundinn hennar = She loves her dog, meaning another woman's dog.

In Icelandic, possessive words often come after the noun, especially forms like minn, þinn, sinn.

So Icelandic says something more like:

• dog the her-own

rather than English her dog.

That word order is completely normal in Icelandic.

Because sinn must agree with the noun it describes.

It agrees in:

• gender
• number
• case

Here it refers to hundinn, which is:

• masculine
• singular
• accusative

So the correct form is sinn.

Other forms appear with different nouns:

• bókina sína = her/his own book
  (bókina is feminine singular accusative definite, so sína is used)
• húsið sitt = her/his own house
  (húsið is neuter singular accusative/nominative definite, so sitt is used)
• hundunum sínum = to her/his own dogs
  (dative plural, so sínum)

Yes, that is its main job.

In a clause, sinn refers back to the subject of that same clause. In this sentence, the subject is hún, so sinn refers back to hún.

That is why the sentence clearly means that she loves her own dog.

This is a very important feature of Icelandic grammar, and English does not mark this distinction as clearly.

It is in the accusative singular masculine.

That is because it agrees with hundinn, which is also accusative singular masculine.

Even though sinn refers back to hún, its form is not feminine just because hún is feminine. Its form is chosen based on the noun it belongs to, not on the subject.

Yes, Icelandic allows more word-order flexibility than English, but the neutral, straightforward order here is:

• Hún elskar hundinn sinn.

That is the most natural basic order for a simple statement: subject + verb + object.

You may see other word orders in special contexts, especially for emphasis or in subordinate clauses, but this sentence as written is the standard version a learner should know first.

You would usually use hennar instead of sinn:

• Hún elskar hundinn hennar.

That means She loves her dog, where her refers to another female person, not the subject herself.

So the contrast is:

• hundinn sinn = her own dog
• hundinn hennar = her dog belonging to another woman

This distinction is one of the most useful things to learn from this sentence.

If you wanted to say She loves her dog in an indefinite sense, you would remove the attached definite article from the noun:

• Hún elskar hund sinn.

Here:

• hund = dog in the accusative singular, without the
• sinn still means her own

So:

• hundinn sinn = her own dog / the dog of hers
• hund sinn = her own dog without the definite article

In practice, which one sounds most natural depends on context.

A rough learner-friendly pronunciation is:

• Hún ≈ hoon
• elskar ≈ EL-skar
• hundinn ≈ HUN-din
• sinn ≈ sin

A few notes:

• ú in hún is like oo in food
• r in elskar is rolled or tapped
• the ll sound is not an issue here, so this sentence is relatively beginner-friendly
• Icelandic spelling is fairly regular, but the exact sounds are best learned by listening to native audio

A natural rhythm would be something like:

HOON EL-skar HUN-din sin