Når først fejlkoden er i emailen, begynder fejlsøgningen.

Here Når først means something like once or once ... then.

So:

• Når først fejlkoden er i emailen, begynder fejlsøgningen.
• = Once the error code is in the email, troubleshooting begins.

The word først is not really first in a simple sequence sense here. It adds the idea that once a certain condition has been met, the next thing follows.

A very useful contrast is:

• Når først ... = once ...
• Først når ... = only when ...

Those are similar, but not identical.

That is because Danish main clauses normally follow the V2 rule: the finite verb comes in the second position.

In this sentence, the whole subordinate clause comes first:

• Når først fejlkoden er i emailen = first element

So in the main clause, the finite verb must come next:

• begynder = second position
• fejlsøgningen = subject comes after the verb

So:

• Når først fejlkoden er i emailen, begynder fejlsøgningen.

If the main clause stood alone, it would be:

• Fejlsøgningen begynder.

But because something else has been placed first, the verb moves ahead of the subject.

Often yes, but not always in exactly the same way.

In Danish, når is commonly used for:

• general truths
• repeated situations
• future situations
• conditions that are expected to happen

In this sentence, it works a lot like when or once in English:

• Når først fejlkoden er i emailen ...
• When/Once the error code is in the email ...

A useful comparison:

• når = when / whenever / once
• da = when, but usually for a specific event in the past

So if you are talking about a general process or rule, når is the natural choice.

Because Danish usually puts the definite article at the end of the noun as a suffix.

Examples from the sentence:

• en fejlkode = an error code

• fejlkoden = the error code

• en email = an email

• emailen = the email

• en fejlsøgning = a troubleshooting process
• fejlsøgningen = the troubleshooting

So instead of a separate word like English the, Danish often adds:

• -en
• -et
• -ene

depending on gender and number.

Fejlsøgning is a noun meaning troubleshooting or fault-finding/debugging, depending on context.

It is built from:

• fejl = error, fault
• søgning = searching

So literally it is something like error-searching.

This is very typical Danish word formation: Danish often combines smaller words into one compound noun.

Likewise:

• fejlkode = error code
  • fejl
    • kode

Compound nouns are extremely common in Danish, just as in German.

Yes, and the meaning can shift a little.

Compare these:

• Når først fejlkoden er i emailen, begynder fejlsøgningen.

  • Once the error code is in the email, troubleshooting begins.

• Når fejlkoden først er i emailen, begynder fejlsøgningen.

  • Very similar; først still gives the sense of once that condition is met.

But this is different:

• Først når fejlkoden er i emailen, begynder fejlsøgningen.
  • Only when the error code is in the email does troubleshooting begin.

That last version is stronger and more restrictive. It emphasizes that troubleshooting does not begin before that point.

So the placement of først matters.

Because Danish often uses the present tense for:

• general rules
• instructions
• routines
• processes
• things that will happen as part of a normal sequence

So:

• er = is
• begynder = begins

This does not have to mean it is happening right now. It can mean:

• this is what normally happens
• this is the next step in the process

English does the same sometimes:

• When the code is in the email, troubleshooting begins.

So the tense is completely natural here.

Yes, emailen is normal and understandable.

You may also see:

• e-mailen
• mailen

All of these occur in real Danish. The choice often depends on style, formality, or personal preference.

Very roughly:

• emailen = common modern spelling
• e-mailen = more traditional spelling
• mailen = very common in everyday speech

So if you meet different versions, that is normal.

Yes, literally in the email, and that is also the natural English translation here.

In practice, i emailen means:

• in the email message
• in the body/content of the email
• included in the email

So the sentence suggests that once the error code has been included in the email, the troubleshooting process can start.

It does not sound strange in Danish at all. It works the same way English uses in the email.