Jeg vil læse et helt afsnit, før jeg går i seng.

Because vil is a modal verb. In Danish, modal verbs such as vil, kan, skal, må, bør are followed by the bare infinitive:

• jeg vil læse
• hun kan komme
• vi skal gå

So after vil, you do not use at.
This is similar to English I will read, not I will to read.

It can suggest either future intention or wanting/intending, depending on context.

So jeg vil læse et helt afsnit can feel like:

• I will read a whole section
• I want to read a whole section
• I’m going to read a whole section

In everyday Danish, vil often expresses intention or willingness, not just simple future. Context tells you which nuance is strongest.

Because afsnit is a neuter noun, so its indefinite singular article is et:

• et afsnit

When an adjective modifies an et-word in the indefinite singular, the adjective usually gets -t:

• et helt afsnit

Compare:

• en hel dag = a whole day
• et helt afsnit = a whole section/paragraph

So:

• hel is the basic form
• helt is used with an et-word
• hele is often used for plural or definite expressions, for example hele dagen or hele afsnittet

Because helt agrees with the neuter singular noun afsnit in an indefinite phrase:

• et helt afsnit

You use hele in different situations, especially with definite nouns or plural nouns:

• hele afsnittet = the whole section
• hele bogen = the whole book
• hele dagen = the whole day
• hele kapitler = whole chapters

So the difference here is grammatical agreement:

• et helt afsnit = indefinite, singular, neuter
• hele afsnittet = definite

This is normal in both Danish and English after time words like før.

Danish often uses the present tense for future events when the time relationship is already clear:

• før jeg går i seng = before I go to bed

English does the same:

• not usually before I will go to bed
• but before I go to bed

So går is present in form, but it refers to a future action.

Because før jeg går i seng is a subordinate clause.

In a main clause, Danish follows the V2 rule: the finite verb usually comes in second position.

• Jeg går i seng.

But after a subordinating conjunction like før, at, fordi, hvis, når, the clause has subject + verb order:

• før jeg går i seng
• fordi jeg er træt
• når vi kommer hjem

So før går jeg i seng would sound wrong here.

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

Even though i often means in/into, here the whole phrase is idiomatic:

• jeg går i seng = I go to bed
• hun gik tidligt i seng = she went to bed early

So this is something you mostly just learn as a set phrase. Danish uses i seng, not til seng, in this expression.

Afsnit is a flexible word. It can mean:

• paragraph
• section
• sometimes part or chapter-like section, depending on context

So the exact English translation depends on what is being read:

• in an article, it may mean paragraph
• in a book or text, it may mean section
• in conversation, it can mean a portion or segment of something

That is why you may see different English translations for the same Danish word.

Because før jeg går i seng is a subordinate clause, and many Danish writers put a comma before subordinate clauses.

So:

• Jeg vil læse et helt afsnit, før jeg går i seng.

is correct.

You may also see:

• Jeg vil læse et helt afsnit før jeg går i seng.

This is also possible in modern Danish, depending on comma style. So the comma before før is common and correct, but you may sometimes encounter the sentence without it.