Jeg ser mig selv i vinduet, når det er mørkt udenfor.

Because mig is the object form of jeg.

In this sentence, jeg is the subject: the person doing the seeing.
The thing being seen is the same person, so Danish uses the object pronoun mig plus selv:

• jeg = I
• mig = me
• mig selv = myself

So:

• Jeg ser mig selv = I see myself

Jeg selv can exist, but it usually means something like I myself for emphasis, not myself as the object.

Yes, you can say Jeg ser mig i vinduet, and it may still be understood.

But selv makes it clearer and more natural that you mean myself specifically, not just me in some looser sense. It adds emphasis and helps mark the reflexive meaning:

• mig = me
• mig selv = myself

So Jeg ser mig selv i vinduet is the clearest way to say I see myself in the window.

Because Danish normally says i vinduet when you mean seeing something in the window or in the glass/window area, especially a reflection.

• i vinduet = in the window
• på vinduet would sound odd here and would suggest something physically on the surface of the window in a less natural way

When talking about reflections, i vinduet is the normal choice.

Vinduet is the definite form: the window.

• et vindue = a window
• vinduet = the window

In this sentence, the speaker is referring to the window they are looking at, so Danish naturally uses the definite form.

This is very common in Danish: where English might sometimes say a, Danish often prefers the definite form when the thing is specific in the situation.

Because når is used for something that happens when/whenever a situation occurs, especially if it is regular or expected.

Here the meaning is:

• when it is dark outside, I see myself in the window

That is a general pattern, so når is the natural choice.

Compare:

• når = when / whenever
• hvis = if

If you said hvis det er mørkt udenfor, it would sound more like a condition: if it is dark outside. That is possible in some contexts, but når fits this sentence better because it describes what normally happens.

The det is a dummy subject, like English it in it is dark.

Danish often uses det with weather, time, light, and general conditions:

• Det er koldt = It is cold
• Det er sent = It is late
• Det er mørkt = It is dark

So det does not refer to a specific thing here. It is just required by the grammar.

Because adjectives in Danish often add -t when used with et-words or in certain predicate constructions like this one.

The base form is:

• mørk = dark

But with det er ..., the usual form is:

• det er mørkt

This is the neuter form of the adjective.

You can think of it as similar to other examples:

• Den er stor = it is big
• Det er stort = it is big

So mørkt is the grammatically correct form after det er here.

In this sentence, udenfor is normally written as one word and means outside.

• udenfor = outside

You may also encounter uden for, but that usually functions differently, more like outside of in relation to something:

• uden for huset = outside the house

In your sentence, mørkt udenfor means dark outside, so udenfor as one word is the normal form.

Because this is a subordinate clause introduced by når.

In Danish main clauses, the verb is usually in second position:

• Jeg ser mig selv i vinduet

But in subordinate clauses, the word order is different, and the subject usually comes before the verb:

• når det er mørkt udenfor

So:

• main clause: Jeg ser ...
• subordinate clause: når det er ...

This is a very important Danish pattern.

Because når det er mørkt udenfor is a subordinate clause.

A comma before a subordinate clause is very common in Danish writing, especially in systems that use start comma.

So the comma helps separate:

• main clause: Jeg ser mig selv i vinduet
• subordinate clause: når det er mørkt udenfor

You may also see variation in Danish comma usage depending on style and comma system, but the comma here is completely normal.

Yes, in normal use it strongly suggests seeing your reflection in the window.

Literally, it means I see myself in the window, but the natural interpretation is that the glass acts like a mirror, especially because of the second part:

• når det er mørkt udenfor = when it is dark outside

When it is dark outside, windows often reflect the inside, so the whole sentence naturally means seeing your reflection.

Danish does not normally use a separate progressive form like English am seeing or am looking in this kind of sentence.

So the simple present ser can cover meanings like:

• I see
• I am seeing
• I do see

depending on context.

Here Jeg ser mig selv i vinduet is the normal Danish way to express the idea. Danish usually relies on context rather than a special progressive verb form.

Yes. That would mean I can see myself in the window.

The difference is:

• Jeg ser mig selv i vinduet = I see myself in the window
• Jeg kan se mig selv i vinduet = I can see myself in the window

Adding kan emphasizes ability or possibility. Without kan, the sentence is more direct and neutral. Both are natural, but the original sentence is perfectly good Danish.