Hann deilir myndbandinu með systur sinni, og hún deilir því svo með vinkonu sinni.

Because deila normally takes the thing being shared in the dative case.

So:

• myndbandið = the video in the nominative/accusative
• myndbandinu = the video in the dative

In this sentence, myndbandinu is the thing being shared, so it appears in the dative:

• Hann deilir myndbandinu ... = He shares the video ...

A very useful pattern to remember is:

• deila einhverju með einhverjum = to share something with someone

Both eitthvað and einhverjum are in the dative in this pattern.

For the same reason: deila requires the shared thing to be in the dative.

The pronoun því refers back to myndbandinu. Since myndband is:

• neuter
• singular
• here in the dative

the pronoun must match that:

• það = nominative/accusative it
• því = dative it

So:

• hún deilir því ... = she shares it ...

This is very common in Icelandic: when you replace a noun with a pronoun, the pronoun keeps the case the verb requires.

Because they come after með, and here með takes the dative.

So:

• systir = sister
• systur = dative form

and

• vinkona = female friend
• vinkonu = dative form

That gives:

• með systur sinni = with his sister
• með vinkonu sinni = with her friend

So in this sentence, there are several dative forms at once:

• myndbandinu
• því
• systur
• vinkonu
• sinni

This is one reason the sentence is a good example of how important case is in Icelandic.

Sinn is a reflexive possessive word. It means his/her/their own, depending on the subject of the clause.

So:

• systur sinni = his own sister
• vinkonu sinni = her own friend

In this sentence:

• Hann deilir myndbandinu með systur sinni
  Here sinni refers back to hann, so it means his sister.

• og hún deilir því svo með vinkonu sinni
  Here sinni refers back to hún, so it means her friend.

If you used hans or hennar instead, it would usually point to someone else, not the subject itself.

So this contrast is important:

• systur sinni = his own sister
• systur hans = his sister but not necessarily reflexive; often someone else’s sister in context

Because sinn behaves like an adjective and must agree with the noun it goes with in:

• gender
• number
• case

Here:

• systur is feminine singular dative
• vinkonu is also feminine singular dative

So the correct form is sinni in both cases.

You can think of it like this:

• dictionary form: sinn
• with feminine singular dative nouns: sinni

So systur sinni and vinkonu sinni are grammatically parallel.

Not necessarily. Grammatically, hún simply means she, and in this sentence it introduces the subject of the second clause.

Because the sentence says:

• Hann ... og hún ...

the most natural reading is:

• first clause: he shares the video with his sister
• second clause: she then shares it with her friend

In normal context, many learners will understand hún as referring to the sister just mentioned, and that is probably the intended meaning. But the pronoun itself does not force that by grammar alone; context tells you who she is.

Here svo means something like:

• then
• after that
• in turn

So:

• hún deilir því svo með vinkonu sinni
  = she then shares it with her friend

It adds a sense of sequence: first he shares it with his sister, then she shares it onward.

This is a very common use of svo in everyday Icelandic.

Yes, svo is somewhat flexible, but the placement here is very natural.

The sentence uses the basic order:

• subject + verb + object/pronoun + svo + prepositional phrase

So:

• hún = subject
• deilir = verb
• því = object
• svo = then
• með vinkonu sinni = with her friend

You may also hear other word orders in Icelandic depending on emphasis, but this version is straightforward and idiomatic.

Yes. Icelandic, like English, often uses a pronoun to avoid repeating the same noun.

So instead of saying:

• ... og hún deilir myndbandinu svo með vinkonu sinni

the sentence says:

• ... og hún deilir því svo með vinkonu sinni

This sounds more natural and less repetitive. Since því clearly refers back to myndbandinu, the meaning stays clear.

The dictionary form is deila, meaning to share.

Deilir is the 3rd person singular present form:

• ég deili = I share
• þú deilir = you share
• hann/hún deilir = he/she shares

So in this sentence:

• Hann deilir ... = He shares ...
• hún deilir ... = She shares ...

Yes. Vinkona specifically means a female friend.

Some related words:

• vinur = male friend / friend
• vinkona = female friend

So með vinkonu sinni means with her female friend.

English usually just says friend, but Icelandic often makes the gender of the person explicit in the noun.

Yes, very much so. It shows a very useful pattern:

• X deilir einhverju með einhverjum
  = X shares something with someone

For example:

• Hún deilir bókinni með vini sínum.
  = She shares the book with her friend.

• Ég deili myndinni með bróður mínum.
  = I share the picture with my brother.

Things to watch when building your own sentences:

1. The thing shared is usually in the dative.
2. After með, the noun is also in the dative here.
3. If you use sinn, it must match the noun and refer back to the subject of its clause.

A native English speaker would probably want to notice these key points:

1. Case matters

   • myndbandinu and því are dative because of deila
   • systur and vinkonu are dative after með

2. Reflexive possession

   • sinni means his/her own
   • it refers back to the subject of each clause

3. Pronouns must match case

   • því is used, not það

4. Sequence word

   • svo means then / after that

5. Natural sentence chaining

   • ..., og hún ... = ..., and she ...

So even though the sentence looks short, it contains several very typical Icelandic structures.