Jeg går først i seng, når jeg har læst et afsnit i bogen.

Here, først does not mean first in the simple sequence sense.

In this sentence, jeg går først i seng, når ... means something like:

• I’m only going to bed when ...
• I won’t go to bed until ...

So først adds the idea of not before that point.

A very literal breakdown would be:

• Jeg går først i seng = I go to bed only then / not until then

This is a very common Danish way to express what English often says with not ... until.

At gå i seng is a fixed Danish expression meaning to go to bed.

So:

• jeg går i seng = I go to bed

You should learn it as a whole phrase. Danish uses i here, not til.

Other examples:

• Hun gik tidligt i seng. = She went to bed early.
• Børnene er gået i seng. = The children have gone to bed.

Yes, når normally means when. But in this sentence, the full structure with først gives the overall meaning not until.

So the sentence is literally closer to:

• I go to bed only when I have read a section in the book

Natural English usually says:

• I won’t go to bed until I’ve read a section of the book

So it is the combination of:

• først = only then / not before
• når = when

that produces the until-type meaning.

Danish often uses the present tense for future meaning, just like English sometimes does.

So:

• Jeg går først i seng ... literally looks present tense
• but it can mean I won’t go to bed ... in a future sense

This is normal when the future is understood from the context.

Compare:

• Jeg kommer i morgen. = I’m coming tomorrow.
• Vi ses senere. = We’ll see each other later.

So går here does not have to mean a habitual action only; it can also describe what will happen.

Har læst is the present perfect:

• har = have/has
• læst = past participle of læse (read)

So:

• jeg har læst = I have read

It is used here because the reading must be completed before the going to bed happens.

The idea is:

• first I finish reading a section
• then I go to bed

So the perfect tense marks that completed action clearly.

Et afsnit usually means a section or a paragraph, depending on context.

In this sentence, it could be understood as:

• a section
• a passage
• sometimes even a chapter, though kapitel is more specifically chapter

Because the sentence says i bogen (in the book), the meaning is probably a section/passsage of text in the book.

Also note:

• et afsnit is a neuter noun
• that is why it uses et, not en

I bogen literally means in the book, and that is natural Danish here.

So:

• et afsnit i bogen = a section in the book

You may also hear af bogen in some contexts, especially if you want to emphasize that something is part of the book:

• et kapitel af bogen = a chapter of the book

But with afsnit, i bogen is very natural because you are thinking of the section as something located inside the book/text.

In Danish, the definite article is often added to the end of the noun instead of being a separate word.

So:

• bog = book
• bogen = the book

This is called the postposed definite article.

Other examples:

• bil = car

• bilen = the car

• hus = house
• huset = the house

So i bogen literally means in-the-book, but in natural English it is simply in the book.

Because når jeg har læst et afsnit i bogen is a subordinate clause.

Danish spelling normally uses commas to separate main clauses and subordinate clauses, so this comma is standard:

• Jeg går først i seng, når jeg har læst et afsnit i bogen.

This is much more regular in Danish than in English, where commas before when are often omitted.

Because når jeg har læst et afsnit i bogen is a subordinate clause, and Danish subordinate clauses usually keep the subject before the finite verb.

So:

• når jeg har læst ... = correct

Not:

• når har jeg læst ...

In a main clause, Danish often has verb-second word order:

• Jeg har læst et afsnit.
• I dag har jeg læst et afsnit.

But after a subordinating word like når, the clause keeps normal subordinate-clause order:

• når jeg har læst ...

Yes, absolutely. That is another very natural way to say it.

• Jeg går først i seng, når jeg har læst et afsnit i bogen.
• Jeg går ikke i seng, før jeg har læst et afsnit i bogen.

Both mean roughly:

• I won’t go to bed until I’ve read a section of the book.

The second version is often more directly parallel to English not ... until.

The version with først ... når is also very common, but learners often need time to get used to it because it does not match English word-for-word.

It could be either, depending on context.

It can mean:

• a habit/routine: I only go to bed once I’ve read a section of the book
• a one-time future action: I won’t go to bed until I’ve read a section of the book

Because Danish uses the present tense here, context decides whether it is habitual or future.

Because there are two clauses, and each clause has its own subject.

Main clause:

• Jeg går først i seng

Subordinate clause:

• når jeg har læst et afsnit i bogen

English does exactly the same:

• I go to bed only when I have read a section of the book

So the second jeg is necessary.

The verb is at læse = to read.

Important forms:

• at læse = infinitive
• læser = present tense
• læste = past tense
• læst = past participle

So:

• jeg læser = I read / I am reading
• jeg læste = I read / was reading
• jeg har læst = I have read

In the sentence, har læst is the perfect form showing that the reading is completed before the next action.