Når der kommer lyn over havet, bliver min datter altid stille med det samme.

Når means when in the sense of something that happens repeatedly or generally.

So Når der kommer lyn over havet ... means Whenever/When there is lightning over the sea ...

A useful contrast is:

• når = when/whenever for repeated or general situations
• da = when for a specific event in the past

So here når is used because this is describing what usually happens, not one single past moment.

In Danish, der is often used in sentences that introduce the existence or appearance of something, similar to English there in there is or there comes.

So:

• der kommer lyn = literally something like there comes lightning
• more naturally: there is lightning / lightning appears

This der is sometimes called an expletive or dummy subject. It does not refer to a place here.

Compare:

• Der står en bil udenfor. = There is a car outside.
• Der kommer regn i morgen. = There will be rain tomorrow.

So der is the normal choice in this kind of structure.

Here lyn is being used in a more general, mass-like sense, similar to English lightning.

So:

• et lyn = a flash of lightning / a lightning bolt
• lyn = lightning in general

In this sentence, the speaker is not talking about one specific lightning bolt, but the phenomenon in general. That is why there is no article.

You can think of it like this:

• Der kommer et lyn. = A lightning bolt appears.
• Der kommer lyn. = There is lightning / lightning is appearing.

Yes, absolutely.

Når det lyner means when it lightnings, which is the natural Danish weather-verb way of saying when there is lightning.

So these are very close in meaning:

• Når der kommer lyn over havet ...
• Når det lyner over havet ...

The version with det lyner is often more compact and very common.
The version with der kommer lyn can feel a bit more visual, as if lightning is appearing over the sea.

Havet is the definite form of hav:

• et hav = a sea
• havet = the sea

Danish often uses the definite form where English might also say the sea, especially for something understood in the situation or treated as a known area.

So over havet means over the sea.

If you said over et hav, that would mean over a sea, which sounds much less natural here unless you are introducing one particular sea among several possibilities.

This is because Danish follows the V2 rule in main clauses: the finite verb usually comes in the second position.

The sentence begins with a subordinate clause:

• Når der kommer lyn over havet

After that, the main clause starts. Because something else is placed first overall, the verb in the main clause comes before the subject:

• ... bliver min datter altid stille med det samme

This is called inversion.

Compare:

• Min datter bliver altid stille med det samme.
• Når der kommer lyn over havet, bliver min datter altid stille med det samme.

Same meaning in the main clause, but the word order changes because of what comes first.

Because bliver means becomes, while er means is.

The sentence is describing a change of state:

• she is not quiet before
• then she becomes quiet

So:

• bliver stille = becomes quiet
• er stille = is quiet

If you said er stille, it would sound more like a description of her state, not the change into that state.

Stille is the normal adjective form here and it does not change in this use.

With Danish adjectives, some adjectives do add -t in the neuter singular, but not all adjectives behave in a simple predictable way in every construction, and stille stays stille here.

Also, after verbs like blive, the adjective is a predicative adjective:

• Hun bliver stille. = She becomes quiet.

That is just the standard form.

So stille is correct exactly as it is.

Med det samme means immediately, right away, or at once.

So:

• bliver min datter altid stille med det samme = my daughter always becomes quiet immediately

It is a very common Danish expression.

Examples:

• Kom med det samme! = Come at once!
• Hun svarede med det samme. = She answered immediately.

Altid means always, and its position follows normal Danish sentence adverb placement.

In a main clause, sentence adverbs like altid, ofte, aldrig often come after the finite verb and subject structure, depending on inversion.

Here the main clause is:

• bliver min datter altid stille med det samme

Because of inversion, the order is:

1. finite verb: bliver
2. subject: min datter
3. adverb: altid
4. complement: stille
5. expression of time/manner: med det samme

If there were no fronted clause, you would get:

• Min datter bliver altid stille med det samme.

So altid is in a very normal position.

Because Danish normally uses a comma before or after subordinate clauses, depending on the sentence structure. Here the subordinate clause comes first:

• Når der kommer lyn over havet, ...

Then the main clause follows:

• ... bliver min datter altid stille med det samme.

So the comma separates the introductory subordinate clause from the main clause.

In modern Danish comma rules, this is standard and helpful for showing the sentence structure clearly.

Yes, it can be.

Komme is often used in Danish for things that appear, arrive, or occur, including weather and natural phenomena.

Examples:

• Der kommer regn. = Rain is coming.
• Der kom tåge i løbet af natten. = Fog came during the night.
• Der kommer lyn over havet. = There is lightning over the sea / lightning appears over the sea.

So even though come in English may sound a bit odd with lightning, in Danish komme works naturally in this type of expression.

Yes. Possessives like min, din, hans, vores already make the noun definite in meaning.

So:

• datter = daughter
• min datter = my daughter

You do not add the Danish definite ending when there is a possessive in front.

That is why it is:

• min datter not
• min datteren

The possessive already does the job.