Når først jeg forstår teksten, bliver resten lettere.

Together, når først means something like once or as soon as in the sense of when that point is finally reached.

Here, først does not mean first in a simple counting sense. It adds the idea that understanding the text is the key step, and after that, the next part becomes easier.

So the sentence suggests:

• understanding the text is the important threshold
• after that threshold, everything else feels easier

Because Danish often uses the present tense after når for something in the future, especially in time clauses.

So:

• Når jeg forstår teksten ... = When/once I understand the text ...

This is very similar to English, which also says:

• When I understand the text, the rest will be easier

  not normally:

• When I will understand the text ...

So the Danish present tense here is completely normal.

This is because Danish main clauses follow the verb-second rule.

The first part of the sentence, Når først jeg forstår teksten, is a subordinate clause. When that whole clause comes first, it counts as the first element of the sentence. Then the finite verb of the main clause must come next:

• Når først jeg forstår teksten, bliver resten lettere.

So the structure is:

• subordinate clause
• finite verb: bliver
• subject: resten

If you removed the fronted clause, you would get:

• Resten bliver lettere.

But once the time clause is placed first, Danish moves the verb up:

• Når først jeg forstår teksten, bliver resten lettere.

This is a very common learner question.

• når is used for when/once, often for something expected, repeated, or treated as a real point in time
• hvis is used for if, for a more open condition

In this sentence, the speaker presents understanding the text as the point after which things get easier. It sounds like a real or expected situation, so når fits well.

Compare:

• Når jeg forstår teksten, bliver resten lettere.
  = once/when I understand it, the rest gets easier

• Hvis jeg forstår teksten, bliver resten lettere.
  = if I understand it, the rest gets easier

The second version sounds more hypothetical.

Yes, very often it works like a set phrase.

Når først ... is a common way to say:

• once ...
• once you finally ...
• after you get to the point where ...

It often introduces a key condition that unlocks the rest of the situation.

So in this sentence, når først is not just when + first translated word for word. It is better understood as a single expression with a special meaning.

Because teksten means the text.

The basic noun is:

• en tekst = a text

Its definite form is:

• teksten = the text

Danish usually puts definiteness at the end of the noun instead of using a separate word like English the.

So:

• jeg forstår teksten = I understand the text

Resten is a noun meaning the rest.

It comes from:

• en rest = a remainder / a rest

Definite form:

• resten = the rest

In this sentence, resten is the subject of the main clause:

• bliver resten lettere = the rest becomes easier

So the sentence is not missing a subject. Resten is the subject.

Because lettere is the comparative form of let.

• let = easy / light
• lettere = easier

The sentence is saying that the rest becomes easier than before, not simply easy.

So:

• bliver lettere = becomes easier

This is exactly the same kind of idea as English easy → easier.

Yes, often you could.

Both can mean easier:

• lettere
• nemmere

In many everyday situations, nemmere is very common when talking about something being easier to do or understand.

But lettere is also completely correct here. It can sound slightly more neutral or formal depending on context.

So:

• bliver resten lettere
• bliver resten nemmere

Both are possible, though the exact nuance can vary a little.

Because the sentence begins with a subordinate clause:

• Når først jeg forstår teksten

Then the main clause follows:

• bliver resten lettere

In Danish, a comma is normally used to separate those parts:

• Når først jeg forstår teksten, bliver resten lettere.

So the comma helps show where the time clause ends and the main statement begins.