Skriv en besked i forvejen, hvis du kommer for sent.

Because skriv is the imperative form of the verb at skrive (to write).

So Skriv ... means Write ... / Send ... as an instruction or request.

• at skrive = to write
• skriver = writes / am writing / are writing
• skriv = write!

In Danish, the imperative is often the verb stem by itself.

Yes. Skriv en besked literally means write a message, and it is completely natural Danish.

In everyday use, en besked can mean:

• a message
• a note
• a text/message sent to someone

Depending on context, English might translate it more naturally as:

• send a message
• text me
• let me know

So even though the verb is write, the overall idea can be broader than physically writing something by hand.

I forvejen means in advance, beforehand, or ahead of time.

So the sentence is telling someone not to wait until they are already late, but to message earlier.

Common equivalents:

• i forvejen = beforehand
• på forhånd = in advance

These are similar, though i forvejen is especially common in everyday speech.

Because Danish usually uses the present tense where English often uses will or going to for future meaning.

So:

• hvis du kommer for sent literally looks like if you come too late
• but in natural English it often means if you are going to be late or if you arrive late

This is very normal in Danish. The future is often understood from context rather than marked explicitly.

At komme for sent is a very common expression meaning to be late or to arrive late.

Literally:

• kommer = come / arrive
• for sent = too late / late

But as a fixed expression, it simply means:

• be late
• arrive late

So you should learn at komme for sent as a useful chunk.

In Danish, for sent is the normal expression for too late / late in this context.

Examples:

• Jeg kommer for sent. = I’m late.
• Bussen kom for sent. = The bus arrived late.

Even though for often means too, in this expression English usually translates it simply as late, not always too late.

So komme for sent is idiomatic Danish and should be learned as a set phrase.

Because hvis introduces a subordinate clause, and in Danish subordinate clauses usually keep the subject before the verb.

So:

• du kommer = you come / you are coming

After hvis (if), that normal subject-verb order stays:

• hvis du kommer for sent

This is different from main-clause inversion patterns you may see elsewhere in Danish.

Because Danish punctuation normally uses a comma before a subordinate clause, and hvis introduces one.

So:

• Skriv en besked i forvejen, hvis du kommer for sent.

That comma helps show that the second part is the condition:

• if you are going to be late

In English, a comma before if is often omitted in a short sentence like this, but in Danish it is standard.

It is grammatically a command because it uses the imperative skriv, but in real life it can function as:

• an instruction
• a request
• practical advice

Tone depends on context.

For example:

• from a boss or teacher, it may sound like an instruction
• from a friend, it may sound like a simple request

Danish often uses the imperative quite naturally without it sounding especially harsh.

Yes, absolutely.

• Skriv en besked = write a message
• Send en besked = send a message

In many real situations, send may sound even more natural in English, especially for a phone message or text. But Danish commonly uses skriv en besked too, and it sounds perfectly normal.

No. En besked is broader than just a text message.

It can mean:

• a text
• a message in an app
• a note
• a message passed on to someone
• information you leave for someone

So the exact kind of message depends on context. In this sentence, it most likely means a quick message, such as a text or chat message.

Yes. That is a very natural way to understand it.

Even though the Danish literally says Write a message in advance, the real communicative meaning is often:

• Let me know in advance if you’re going to be late
• Send a message beforehand if you’ll be late

That kind of freer translation is often the most natural in English.