Giv lyd, når du er hjemme igen.

Yes. Giv lyd is an idiomatic expression.

• giv = give (imperative of at give)
• lyd = sound

So the literal meaning is something like give sound, but that is not how it works in natural English. In Danish, give lyd means let me hear from you, get in touch, or let me know.

It is a fairly common expression, though it can sound a little more idiomatic than something very neutral like giv besked.

Because this is a command/request, so Danish uses the imperative.

• infinitive: at give
• imperative: giv

Just like in English:

• Give me a call
• Let me know

you do not normally include the subject you. Danish works the same way here.

So:

• Giv lyd = Get in touch / Let me know

not

• Du giver lyd = You make a sound / you get in touch (a statement, not a command)

In Danish imperatives, the subject is usually left out, just as in English.

Compare:

• English: Come here

• Danish: Kom her

• English: Call me
• Danish: Ring til mig

So Giv lyd is completely normal. The you is understood.

If you did include du, it would usually no longer sound like a simple imperative.

Når is used for when in cases where something is expected to happen.

Here, the speaker assumes that the other person will be home again at some point, so:

• når du er hjemme igen = when you are home again

This is different from:

• hvis = if, when something is uncertain
• da = often when about a specific past event, or since/because in some contexts

So here når is the natural choice because the return home is treated as expected, not hypothetical.

Both are possible, but they focus on slightly different moments.

• når du kommer hjem igen = when you come home again
  • focuses on the arrival
• når du er hjemme igen = when you are home again
  • focuses on the state of being back home

In practice, both can work in many situations, but er hjemme igen can sound a little more like once you’re back home.

This is a very common question for English speakers.

• hjem usually expresses movement toward home
• hjemme usually expresses being at home

So:

• Jeg går hjem = I’m going home
• Jeg er hjemme = I’m at home

In your sentence, it is er hjemme because the verb er describes a location/state, not movement.

Igen means again.

So:

• hjemme igen = home again / back home

It adds the idea that the person was home before, then away, and is now returning to that state.

Without igen:

• Giv lyd, når du er hjemme = Let me know when you’re home

With igen:

• Giv lyd, når du er hjemme igen = Let me know when you’re back home

Yes, it is a subordinate clause, introduced by når.

In Danish subordinate clauses, the word order is typically more straightforward than in main clauses:

• når du er hjemme igen

This is similar to English order: when you are home again.

A useful contrast:

• Main clause: Du er hjemme igen
• Subordinate clause: når du er hjemme igen

In this example, the order looks the same, but in Danish, subordinate clauses are important because adverbs and sentence structure often behave differently from main clauses.

Giv lyd is natural, but it is somewhat idiomatic. A very common neutral alternative is:

• Giv besked, når du er hjemme igen.

That means something like Let me know when you’re back home.

You could also hear:

• Skriv, når du er hjemme igen. = Text/write when you’re back home
• Ring, når du er hjemme igen. = Call when you’re back home

So giv lyd is fine, but giv besked is often the safest everyday equivalent for learners.

A careful pronunciation would be roughly:

• Giv lyd → something like geev lyyth
• når du er hjemme igen → something like nor du er yeme i-gen

A few useful notes:

• y in lyd is a rounded vowel that English does not really have
• d in Danish is often softer than English d
• hjemme begins with hj-, but in modern pronunciation the j is not pronounced like a strong English y in all contexts; the word often sounds smoother than spelling suggests

If you want to sound natural, it is best to learn this phrase by listening to native audio, because Danish pronunciation is often less phonetic than the spelling suggests.