Det virker underligt, at lyset tænder af sig selv om natten.

Here det is a dummy subject or formal subject. It does not refer to a specific thing like the light.

In Danish, just as in English, you often use det / it in expressions like:

• Det er mærkeligt, at ... = It is strange that ...
• Det virker underligt, at ... = It seems strange that ...

So det is there because Danish normally wants a subject in the sentence, even when the real content comes later in the at-clause.

In this sentence, virker means seems or appears, not works.

The verb virke can mean different things depending on context:

• Maskinen virker. = The machine works.
• Det virker underligt. = It seems strange.

So in your sentence, Det virker underligt means It seems strange.

Underlig is the basic adjective form, but here it becomes underligt because it is used with det and with a clause as the thing being judged.

Compare:

• En underlig lyd = a strange sound
• Det er underligt = it is strange
• Det virker underligt = it seems strange

This -t form is the neuter / adverb-like predicative form often used after det er, det virker, det lyder, etc.

At here means that and introduces a subordinate clause.

So:

• Det virker underligt = main clause
• at lyset tænder af sig selv om natten = subordinate clause explaining what seems strange

This is very similar to English:

• It seems strange that the light turns on by itself at night.

In everyday English, that is sometimes omitted, but in Danish at is normally kept in this kind of sentence.

Lys means light as a noun, and lyset means the light.

The noun lys is a neuter noun in Danish, so its definite singular form ends in -et:

• et lys = a light / a candle / a light source
• lyset = the light

Here the sentence is talking about a specific light, so Danish uses the definite form lyset.

This is a very natural question.

In Danish, tænde can be used both:

• transitively: someone turns something on
  
  • Jeg tænder lyset. = I turn on the light.
• intransitively: something turns on
  
  • Lyset tænder. = The light turns on.

So lyset tænder means the light turns on.

Other versions are possible, but they are slightly different:

• Lyset tændes = the light is turned on / the light turns on
  This sounds more passive or formal.
• Lyset bliver tændt = the light gets turned on
  This strongly suggests some action is done to it.

Because the sentence says af sig selv (by itself), the active intransitive lyset tænder sounds quite natural.

Af sig selv means by itself, on its own, or without anyone doing it.

Examples:

• Døren åbnede af sig selv. = The door opened by itself.
• Computeren slukkede af sig selv. = The computer shut off by itself.

In your sentence, it emphasizes that no person is turning the light on.

A useful detail: sig is the reflexive pronoun used when the subject refers back to itself.

Om natten is the normal Danish expression for at night.

This is an idiomatic time expression:

• om dagen = during the day / in the daytime
• om natten = at night
• om morgenen = in the morning

I natten would usually not sound natural for this meaning. It would sound more literal or poetic, like in the night, not the ordinary time expression at night.

Also, om natten often suggests something habitual or typical, not just one single night.

The subordinate clause has normal subordinate-clause word order:

• at
  • subject + verb + other elements

So:

• at = subordinator
• lyset = subject
• tænder = verb
• af sig selv = adverbial phrase
• om natten = time expression

Unlike main clauses, Danish subordinate clauses do not use the usual verb-second pattern.

Compare:

• Main clause: Lyset tænder om natten.
• Subordinate clause: ... at lyset tænder om natten.

In this sentence, the order is straightforward and neutral.

Yes, they can, and the meaning would stay very similar:

• ... at lyset tænder af sig selv om natten
• ... at lyset tænder om natten af sig selv

But the first version sounds more natural and smoother.

A common tendency in Danish is to keep af sig selv close to the verb because it describes how the turning-on happens, while om natten gives the time.

So the original order is the most idiomatic.

Because at lyset tænder af sig selv om natten is a subordinate clause, and Danish normally places a comma before such clauses.

So:

• Det virker underligt, at lyset tænder af sig selv om natten.

This is standard Danish punctuation.

Usually om natten means at night / during the night in a general or repeated sense.

So the sentence often suggests something like:

• this happens at night
• it is a nighttime behavior
• it tends to happen during the night

If you wanted to refer to tonight, Danish would usually say i nat instead.

Compare:

• Lyset tænder om natten. = The light turns on at night.
• Lyset tænder i nat. = The light turns on tonight.

Yes, it is natural and understandable Danish.

A very close alternative would be:

• Det virker underligt, at lyset tænder af sig selv om natten.

You could also hear:

• Det er underligt, at lyset tænder af sig selv om natten.
• Det virker mærkeligt, at lyset tænder af sig selv om natten.

These are all natural, with small differences in tone:

• virker = seems
• er = is
• underligt / mærkeligt = strange / odd