Sæt lygten fast på cyklen, før du kører hjem i mørket.

Sæt is the imperative form of at sætte (to put / to set / to attach).

In Danish, commands often begin directly with the imperative verb:

• Sæt lygten fast = Attach the light
• Kom her = Come here
• Vent lidt = Wait a moment

So the sentence begins with a direct instruction.

Here, sætte ... fast means to fasten / attach securely.

So:

• Sæt lygten fast = Fasten the light
• not just put the light somewhere, but make sure it is fixed firmly in place

The word fast often means firmly, securely, fixed in this kind of expression.

Lygten is the definite form of lygte, meaning the light / the lamp.

In Danish, definiteness is often shown by adding an ending to the noun:

• en lygte = a light
• lygten = the light

Here, lygten is used because it refers to a specific light, probably the one that belongs on the bicycle or the one both speaker and listener already know about.

In this sentence, lygte means a light or lamp, especially a practical light such as a bike light.

So lygten here is best understood as the bicycle light, not something decorative like a household lamp.

På cyklen means on the bicycle.

• på = on
• cyklen = the bicycle

In this sentence, it means the light should be attached onto the bike.

Danish often uses på where English would also use on:

• på bordet = on the table
• på væggen = on the wall
• på cyklen = on the bike

Just like lygten, cyklen is a definite noun:

• en cykel = a bicycle
• cyklen = the bicycle

The sentence is talking about a specific bicycle, most likely your bike or the bike being used.

Før means before.

It introduces a time clause:

• før du kører hjem = before you go/ride home

So the second part tells you when you should fasten the light: before going home in the dark.

Because før du kører hjem i mørket is a subordinate clause.

In Danish, main clauses usually follow the V2 pattern, where the finite verb is in second position. But subordinate clauses usually do not use inversion; they normally keep the order:

subject + verb

So:

• du kører = you go / you ride
• not kører du, which would be used in a question or after fronting in a main clause

Compare:

• Du kører hjem. = You’re going home.
• før du kører hjem = before you go home

Kører comes from at køre, which has a broad meaning in Danish. It can mean:

• drive
• ride
• go by vehicle
• sometimes simply travel/go

Because the sentence mentions cyklen, the most natural English meaning here is ride:

• før du kører hjem = before you ride home

But in other contexts, køre could refer to driving a car, taking a train, or moving by vehicle more generally.

Hjem means home, and in Danish it often works as an adverb of direction, just like in English.

So:

• køre hjem = ride/go home
• gå hjem = walk home
• kom hjem = come home

You usually do not need a preposition before hjem when it means movement toward home.

I mørket literally means in the darkness or in the dark.

• i = in
• mørket = the darkness / the dark

In natural English, in the dark is the best translation here.

It tells us the condition under which the person is going home: when it is dark outside.

Mørket is the definite form:

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

Danish often uses the definite form in expressions where English also says the dark:

• i mørket = in the dark

This is a very natural expression in Danish.

Not quite. That English version can sound as if you are only placing it there temporarily.

Because of fast, the Danish sentence specifically means:

• fasten the light onto the bike
• attach the light securely to the bike

So Put the light on the bike is too weak unless the context makes clear that it means attaching it properly.

Yes. The sentence sounds like a normal practical instruction, especially about biking safely in the dark.

It has a natural everyday tone:

• Sæt lygten fast på cyklen = practical command
• før du kører hjem i mørket = safety-related time condition

A native speaker would understand it as sensible advice or an instruction, especially because bike lights are important in Denmark.