Sørg for, at lygten sidder fast på cyklen, før du kører hjem.

Sørg for, at is a very common Danish way to say make sure that.

• sørg is the imperative of at sørge = to make sure / to see to / to take care
• for is part of the fixed expression
• at introduces the clause that follows

So the pattern is:

• Sørg for, at + clause
• Make sure that + clause

In this sentence:

• Sørg for, at lygten sidder fast ...
• Make sure that the light is securely attached ...

You should learn sørge for, at as a chunk, because it is very idiomatic.

Because at lygten sidder fast på cyklen is a subordinate clause.

Danish often places a comma before subordinate clauses, especially in formal standard writing. Here:

• main clause: Sørg for
• subordinate clause: at lygten sidder fast på cyklen

So the comma marks the boundary between them.

You also see another comma before før, because før du kører hjem is another subordinate clause.

No. Danish at can do two different jobs.

1. Infinitive marker

   • at køre = to ride / to drive

2. Conjunction meaning that

   • Sørg for, at lygten sidder fast = Make sure that the light is attached securely

In your sentence, at is a conjunction, not an infinitive marker.

Because lygten means the light, while en lygte means a light.

Danish often uses the definite form when talking about a specific object that is understood from the situation. If someone is getting ready to ride a bike, lygten naturally means the bike light.

• en lygte = a light
• lygten = the light

The definite form is made by adding an ending:

• lygte → lygten

That is very common in Danish.

Sidder fast means something like:

• is firmly attached
• is secure
• is stuck in place

Literally, sidder means sits, and fast means firmly / tightly / stuck. But together, sidde fast is a normal Danish expression for something being fixed in place.

So:

• Lygten sidder fast på cyklen
• The light is firmly attached to the bike

This is more idiomatic than translating word-for-word.

Danish often uses verbs like sidde, stå, and ligge where English would simply use be.

These verbs can describe position or how something is placed.

So:

• sidde fast suggests that the light is mounted/attached in a secure position
• være fast would sound less natural here

Compare:

• Billedet hænger på væggen = The picture is hanging on the wall
• Nøglerne ligger på bordet = The keys are on the table
• Lygten sidder fast på cyklen = The light is securely attached to the bike

For an English speaker, this is a very useful Danish habit to notice.

Because på is the natural preposition here for something attached on a bicycle.

• på cyklen = on the bike

Danish uses på with many things that are on, attached to, or located on a surface/object.

Here, the bike light is physically mounted on the bicycle, so på cyklen is the normal choice.

Both are possible, but cyklen is very natural when the context already makes it clear whose bike it is.

Danish often uses the definite form where English might prefer a possessive:

• Tag jakken på = Put on your jacket
• Han brækkede benet = He broke his leg
• Sørg for, at lygten sidder fast på cyklen = Make sure the light is attached to the bike

If you said din cykel, it would sound more explicit:

• Sørg for, at lygten sidder fast på din cykel

That is not wrong, just a little less neutral in this context.

Because Danish often uses the present tense to talk about the future when the time is already clear from the context.

Here, før already places the action in the future relative to the command.

So:

• før du kører hjem literally: before you ride/go home
• natural English: before you head home

This is very normal Danish:

• Vi ses i morgen = We’ll see each other tomorrow
• Jeg ringer, når jeg kommer hjem = I’ll call when I get home

So kører here is present in form, but future in meaning.

Because før introduces a subordinate clause, and Danish subordinate clauses normally keep the subject before the verb.

So the pattern is:

• før du kører hjem
• before you go home

That is standard subordinate clause word order.

In main clauses, Danish often has verb-second word order, but subordinate clauses do not work the same way.

Compare:

• main clause: Du kører hjem nu
• subordinate clause: før du kører hjem

Not always, but in this sentence that is the most natural interpretation because the sentence mentions a bicycle and its light.

The verb køre is broader than English drive. It can be used for traveling by vehicle, and in context it can also fit riding a bike.

So here:

• du kører hjem most naturally means you ride home

But in other contexts, køre hjem could mean:

• drive home
• go home by vehicle

Context decides the best English translation.

In standard Danish, no. You normally need at after sørg for when a full clause follows.

Correct:

• Sørg for, at lygten sidder fast på cyklen

Not standard:

• Sørg for, lygten sidder fast på cyklen

So the structure is:

• sørge for noget = make sure of something
• sørge for, at + clause = make sure that ...

Not always. Før can be used in more than one way.

1. Before + clause

   • før du kører hjem = before you go home

2. Before + noun

   • før hjemturen = before the trip home

In your sentence, it introduces a clause with a subject and verb:

• du kører

So før means before and starts a subordinate clause.