Jeg ser efter, om refleksen sidder fast, før jeg cykler i mørket.

Because at se efter is a fixed expression meaning to check, to look and see whether something is okay, or to make sure.

On its own, ser usually just means see or look.
So:

• Jeg ser. = I see / I am looking
• Jeg ser efter ... = I check / I look to see ...

In this sentence, Jeg ser efter, om ... means something like I check whether ...

Here, om means whether / if.

It introduces an indirect yes-no question:

• Jeg ser efter, om refleksen sidder fast.
• I check whether the reflector is attached securely.

This is different from other meanings of om, such as:

• about: en bog om Danmark
• in / after (time): om en time
• around: gå om hjørnet

So in this sentence, om specifically means whether.

In Danish, commas are commonly used before subordinate clauses, and om refleksen sidder fast is a subordinate clause.

So the structure is:

• main clause: Jeg ser efter
• subordinate clause: om refleksen sidder fast

Danish uses commas more regularly in places where English sometimes does not. So even if the English translation might not always include a comma, Danish often does.

This is very natural Danish. At sidde fast means to be attached firmly, to be stuck, or to be fixed in place.

Danish often uses position verbs like:

• sidde = sit
• stå = stand
• ligge = lie

These are used more broadly than in English.

So refleksen sidder fast does not mean the reflector is literally sitting like a person. It means it is properly attached.

This is a very common Danish way to talk about how things are physically positioned or fixed.

Refleksen is the definite form, meaning the reflector.

Danish often adds definiteness to the end of the noun:

• en refleks = a reflector
• refleksen = the reflector

In this sentence, the speaker is talking about a specific reflector, probably the one on their bike, clothes, or bag. That is why the definite form is used.

It is a common-gender noun in the singular definite form.

The forms are:

• en refleks = indefinite singular
• refleksen = definite singular
• reflekser = indefinite plural
• reflekserne = definite plural

A learner may also notice that -en is the usual definite ending for many en-words.

Because subordinate clauses in Danish normally do not use the same inversion pattern as main clauses.

In a main clause, Danish is a V2 language, so the finite verb often comes early:

• Refleksen sidder fast.

But after a subordinator like om, the subject normally comes before the verb:

• om refleksen sidder fast

So this is standard subordinate clause word order:

• om
  • subject + verb + rest

Because før on its own can introduce a subordinate clause meaning before. You do not need at here.

So:

• før jeg cykler i mørket = before I cycle in the dark

Using at after før here would be ungrammatical in standard Danish.

Danish often uses the present tense for:

• habits
• general routines
• near future
• future events that are understood from context

So før jeg cykler i mørket can mean:

• before I cycle in the dark as a habit/routine
• before I go cycling in the dark in a specific situation

This is very normal and similar to English in sentences like:

• I check it before I leave.

Even though the leaving may happen shortly afterward, the present tense is still used.

I mørket means in the dark / in the darkness and is a common fixed expression.

Here mørket is the definite form of mørke:

• mørke = darkness / dark
• mørket = the darkness / the dark

In many cases, Danish uses the definite form where English also tends to say the dark:

• i mørket = in the dark

So this is just the natural idiomatic form.

Yes. Før introduces a subordinate clause.

So the sentence contains two subordinate clauses:

• om refleksen sidder fast
• før jeg cykler i mørket

The overall structure is:

• main clause: Jeg ser efter
• subordinate clause 1: om refleksen sidder fast
• subordinate clause 2: før jeg cykler i mørket

This is one reason the sentence may feel a little dense to a learner, even though it is quite natural Danish.

Yes, in many contexts you could say Jeg tjekker, om refleksen sidder fast.

That would also mean I check whether the reflector is securely attached.

However, ser efter often sounds a bit more everyday and traditionally Danish, while tjekker is a more modern loanword from English. Both are common, but ser efter is a very useful expression to learn because it appears often in ordinary speech.

No. In this sentence, fast means firmly, securely, or stuck/attached.

So:

• sidder fast = is attached firmly / is stuck / is secure

This is a very common meaning of fast in Danish. It often corresponds to English stuck or securely fixed, not quickly.

Probably yes from context, but grammatically it does not have to be. Refleks can refer to a reflector or reflective item, for example on:

• a bicycle
• clothing
• a bag
• safety gear

Because the sentence also says før jeg cykler i mørket, many learners will naturally assume it is a reflector connected with cycling, but the grammar itself does not force only one interpretation.