Hún kastar boltanum til vinkonu sinnar.

Kasta is the dictionary form, meaning to throw.

In the sentence, kastar is the present-tense form used with hún (she):

• ég kasta = I throw
• þú kastar = you throw
• hún kastar = she throws

So Hún kastar means She throws.

Because the verb kasta usually takes its object in the dative case.

The noun bolti means ball, but in this sentence it appears as boltanum, which is the dative singular definite form: the ball.

So:

• bolti = a ball / ball (nominative)
• boltinn = the ball (nominative)
• boltanum = the ball (dative)

The reason for the dative is the verb:

• að kasta einhverju = to throw something

Here, einhverju is dative, so boltanum is too.

In Icelandic, the is often attached to the end of the noun instead of being a separate word.

So boltanum contains both:

• the noun bolt-
• the definite meaning the
• the dative ending

That is very common in Icelandic. Instead of a separate word like English the, Icelandic usually uses a suffixed definite article.

Because the preposition til requires the genitive case.

So after til, the noun has to be in genitive:

• vinkona = female friend (dictionary form)
• til vinkonu = to a female friend / to the female friend

With this particular noun, several singular forms happen to look the same, so vinkonu can be accusative, dative, or genitive singular depending on context. Here it is genitive because of til.

Not always. Til has a range of meanings, depending on context. Common ones include:

• to / toward
• for
• until
• for the purpose of

In this sentence, til means something like to or toward:

• Hún kastar boltanum til vinkonu sinnar.
  = She throws the ball to her friend.

So you should learn both the basic meaning and the fact that til takes the genitive.

Because Icelandic uses a special reflexive possessive when the possessor is the same as the subject of the sentence.

Here, the subject is hún (she), and the friend belongs to that same person. So Icelandic uses sinn:

• vinkonu sinnar = her own friend

If you said vinkonu hennar, it would usually mean the friend of some other woman/girl, not the subject herself.

So this is an important distinction:

• sinnar = her own
• hennar = her, someone else’s

Because sinn changes form to match the noun it belongs to.

It agrees with the possessed noun in:

• gender
• number
• case

Here it goes with vinkonu, which is:

• feminine
• singular
• genitive

So the correct form is sinnar.

A key point: sinn agrees with the thing possessed, not directly with the owner.

Vinkona specifically means a female friend.

Compare:

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

So the sentence tells you that the friend is female.

The sentence uses a very normal Icelandic word order:

• Hún = subject
• kastar = verb
• boltanum = object
• til vinkonu sinnar = prepositional phrase

So structurally it is:

Subject + Verb + Object + Prepositional phrase

That is the most neutral, straightforward way to say it.

Yes, that is a useful literal way to think about it.

Because of til, the sentence can feel a bit like:

• She throws the ball to her friend
• She throws the ball toward her friend

In natural English, to her friend is usually the best translation, but the more literal sense can help you remember how til works.

Here are the main forms you would usually look up:

• hún = she
• kasta = to throw
• bolti = ball
• til = to, toward; also other meanings depending on context
• vinkona = female friend
• sinn = reflexive possessive: one’s own, his/her/their own

This is helpful because the forms in the sentence are often inflected, so they may not look exactly like the dictionary entry.