Hon satte ett bokmärke i sista kapitlet, eftersom hon ville berätta för sin vän vad huvudpersonen faktiskt valde.

Satte is the past tense of sätta. In this sentence, it means put / placed / inserted.

So:

• sätta = to put, set, place
• satte = put/placed

A common confusion is with sitta (to sit). That verb has the past form satt, not satte.

Sometimes yes, but satte ett bokmärke sounds very natural when you mean putting a bookmark into position in the book.

Very roughly:

• lägga = lay/put (more general)
• sätta = set/place/put into position

So hon satte ett bokmärke i sista kapitlet suggests she deliberately placed the bookmark there.

Because bokmärke is an ett-word (a neuter noun).

Its forms are:

• ett bokmärke = a bookmark
• bokmärket = the bookmark
• bokmärken = bookmarks

Also, bokmärke is a compound noun:

• bok = book
• märke = mark/sign/token

In Swedish compounds, the last part determines the gender, and märke is an ett-word, so bokmärke is too.

Because Swedish normally says i ett kapitel / i kapitlet for something located in a chapter.

So:

• i kapitlet = in the chapter
• på sidan = on the page

Using på with kapitel would sound unnatural here. A chapter is understood as a section inside the book, so i is the normal choice.

Because kapitlet is definite: the chapter. In Swedish, adjectives before definite nouns usually take the weak form, and that is sista here.

So:

• ett sista kapitel = a last chapter
• det sista kapitlet = the last chapter

In this sentence, i sista kapitlet means in the last chapter. Swedish often omits det with words like sista, especially in phrases like this, so:

• i sista kapitlet
• i det sista kapitlet

are both possible.

But sistat is not correct here.

Because eftersom introduces a subordinate clause.

In Swedish subordinate clauses, the subject normally comes before the finite verb:

• eftersom hon ville = because she wanted

Swedish only does the typical verb-second inversion in main clauses. Compare:

• Main clause: Därför ville hon berätta ...
• Subordinate clause: eftersom hon ville berätta ...

So after eftersom, hon ville is the normal order.

Because vilja is followed by a bare infinitive.

So you say:

• ville berätta = wanted to tell

not

• ville att berätta

This is similar to other Swedish verbs such as:

• kan läsa = can read
• ska gå = will/is going to go
• måste arbeta = must work

With berätta, Swedish usually uses för before the person who receives the information.

So the pattern is:

• berätta något för någon = tell someone something

Here:

• berätta för sin vän vad ... = tell her friend what ...

This is different from English, where you can just say tell someone without a preposition.

A useful comparison:

• berätta för någon = tell someone
• berätta om något = tell about something

Because sin/sitt/sina is the reflexive possessive. It refers back to the subject of the same clause.

In eftersom hon ville berätta för sin vän ..., the subject is hon, so sin vän means her own friend.

Compare:

• hon berättade för sin vän = she told her own friend
• hon berättade för hennes vän = she told her friend, but her refers to some other female person

So sin is the natural choice when the friend belongs to the subject of that clause.

Because this is an embedded question (or content clause), not a direct question.

Direct question:

• Vad valde huvudpersonen? = What did the main character choose?

Embedded question:

• ... vad huvudpersonen valde = ... what the main character chose

In Swedish, embedded questions do not use the same inversion as direct questions. So after vad, you keep normal subordinate-clause word order:

• vad huvudpersonen valde

not

• vad valde huvudpersonen

Here faktiskt means something like actually / in fact / really.

In subordinate clauses, adverbs like this often come before the finite verb, so:

• vad huvudpersonen faktiskt valde

is natural.

Compare a main clause:

• Huvudpersonen valde faktiskt ...

So the placement changes with clause type.

Huvudpersonen is a compound noun:

• huvud = head/main
• person = person

Together, huvudperson means main character / protagonist.

Then -en makes it definite:

• en huvudperson = a main character
• huvudpersonen = the main character

So vad huvudpersonen faktiskt valde means what the main character actually chose.