I den stille kupé læser min veninde videre i sin roman.

Because Danish uses verb-second (V2) word order in main clauses.

Here, the sentence starts with the adverbial phrase I den stille kupé. Once that element takes the first position, the finite verb must come next:

• I den stille kupé = first position
• læser = second position
• min veninde = subject, placed after the verb

So the structure is:

I den stille kupé + læser + min veninde + ...

If you started with the subject instead, you would get a more basic word order:

Min veninde læser videre i sin roman i den stille kupé.

Because kupé is a common-gender noun in Danish, not a neuter noun.

Danish singular nouns are usually either:

• common gender → take en / den
• neuter → take et / det

Since it is en kupé, the definite form with an added adjective uses den:

• en stille kupé = a quiet compartment
• den stille kupé = the quiet compartment

If the noun were neuter, you would use det instead.

That is the normal adjective form when an adjective comes before a definite noun phrase.

Compare:

• en stille kupé = an indefinite noun phrase
• den stille kupé = a definite noun phrase

After den/det/de, adjectives usually take -e:

• den stille kupé
• det store hus
• de gamle bøger

So stille here is the expected form in a definite phrase.

They do two different jobs.

1. I den stille kupé
   Here i means in and introduces a place: in the quiet compartment.

2. i sin roman
   Here i belongs to the expression læse i noget, which means to read in something or to be reading from/in a book, magazine, novel, etc.

So Danish often says:

• læse i en bog
• læse i avisen
• læse videre i sin roman

Even though English may simply say read her novel or keep reading her novel, Danish often uses i with reading material.

Videre means something like further, on, or to continue.

So læser videre means:

• continues reading
• reads on
• keeps reading

It adds the idea that the action is already in progress and continues.

You will also see videre with other verbs, where it often has the sense of moving or continuing onward.

Because sin/sit/sine is the reflexive possessive used when the possessor is the subject of the clause.

In this sentence, the subject is min veninde, so if the novel belongs to her, Danish normally uses sin:

• Min veninde læser i sin roman = my female friend is reading in her own novel

If you said hendes roman, it would usually suggest that the novel belongs to some other woman, not the subject.

This is a very important distinction in Danish:

• sin/sit/sine = his/her/their own, referring back to the subject
• hans/hendes/deres = someone else’s, or not reflexive

Because the form of the reflexive possessive depends on the gender and number of the thing owned, not on the owner.

The noun being possessed here is roman, which is:

• singular
• common gender

So the correct form is sin.

Compare:

• sin roman = her/his own novel
• sit hus = her/his own house
• sine bøger = her/his own books

Veninde specifically means a female friend.

Danish distinguishes:

• ven = male friend, or sometimes friend in a general sense depending on context
• veninde = female friend

So min veninde clearly tells you the friend is female.

Because after a possessive like sin, Danish normally uses the noun in its indefinite form.

So:

• sin roman = her/his own novel
• min bog = my book
• deres hus = their house

You generally do not combine a possessive with the suffixed definite form in standard Danish.

So sin roman is the normal structure, not sin romanen.

Yes. A perfectly normal version would be:

Min veninde læser videre i sin roman i den stille kupé.

That word order is more neutral because it begins with the subject.

The original sentence starts with I den stille kupé to put focus on the location. Danish allows this kind of fronting, but when you do it, the verb must still stay in second position:

• Min veninde læser ...
• I den stille kupé læser min veninde ...

Both are grammatical; they just highlight different things.

It simply means in the quiet compartment. There is no special case ending here like you might find in German.

Modern Danish has very little case marking in nouns. Instead, relationships are mostly shown by:

• word order
• prepositions
• articles and possessives

So i marks the location, and den stille kupé is just the noun phrase after the preposition.