Vinduet lyser rummet op med måneskin.

Because Danish usually puts the definite article onto the end of the noun instead of using a separate word like the.

• et vindue = a window
• vinduet = the window
• et rum = a room
• rummet = the room

Both vindue and rum are neuter nouns, so their definite ending is -et.

A useful extra point: if there is an adjective, Danish often uses both a separate article and a changed adjective form, for example det store vindue = the big window.

The basic form is at lyse, which means to shine, to give light, or in some contexts to illuminate.

Here, lyser is the present tense:

• at lyse = to shine / to light
• lyser = shines / lights

Danish present tense is very simple: it usually ends in -r, and it does not change for different persons.

• jeg lyser
• du lyser
• vinduet lyser

All use the same form lyser.

Lyse op means to light up or to illuminate.

So:

• lyser rummet op = lights up the room

This is more specific than just lyser on its own. The particle op adds the sense of making something bright or illuminated.

Compare:

• Vinduet lyser = The window shines / gives off light
• Vinduet lyser rummet op = The window lights up the room

Because lyse op is a particle verb (similar to an English phrasal verb like light up), and in a normal Danish main clause the finite verb comes early in the sentence, while the particle often appears later.

So the full verb is:

• at lyse op

But in the sentence you get:

• Vinduet lyser rummet op

That split is completely normal in Danish.

Because with many Danish particle verbs, the direct object is placed between the verb and the particle.

So the normal pattern is:

• lyse noget op = light something up
• skrive noget ned = write something down

That gives:

• lyser rummet op = lights up the room

So op still belongs to the verb, but the object rummet is inserted before it.

Med måneskin means with moonlight.

Here, med shows the means or material involved: the room is being lit with / by means of moonlight.

So the phrase tells you what kind of light is lighting the room.

You could think of it as:

• using moonlight
• by moonlight
• with moonlight

Depending on translation style, all of those can fit the idea.

Because måneskin is being used as a mass noun in a general sense, like water, sunlight, or fog.

So:

• med måneskin = with moonlight

This is the normal way to talk about moonlight as a substance or phenomenon in general.

If you said måneskinnet, that would mean the moonlight, referring to a more specific or already identified moonlight.

Compare:

• Rummet er fyldt med måneskin = The room is filled with moonlight
• Måneskinnet fra i nat var smukt = The moonlight from tonight was beautiful

Yes, it is understandable and grammatically fine, but it sounds a bit literary or poetic.

The slightly unusual part is that the window is treated as the thing that lights up the room. In very literal everyday speech, a Dane might be more likely to say something like:

• Måneskinnet lyser rummet op gennem vinduet
  The moonlight lights up the room through the window

or

• Der kommer måneskin ind gennem vinduet og lyser rummet op
  Moonlight comes in through the window and lights up the room

So the original sentence works well if the image or mood is important.

By itself, lyse can certainly mean shine or glow.

But in this sentence, because it has:

• a direct object: rummet
• and the particle op

the meaning is clearly light up / illuminate, not just shine.

So here:

• Vinduet lyser rummet op
  does not mean merely The window shines
  It means The window lights up the room

In this sentence, the word order strongly suggests it.

Danish main clauses usually follow a verb-second pattern. Here the structure is:

• Vinduet = subject
• lyser = finite verb
• rummet = direct object
• op = particle
• med måneskin = prepositional phrase

So the sentence is built like:

[Subject] [Verb] [Object] [Particle] [Extra information]

That is why the natural reading is:

• The window lights up the room with moonlight

not the other way around.