Cykellåsen ligger i tasken, men jeg sætter den altid fast om hjulet ved skolen.

Cykellåsen is the definite form of en cykellås.

• en cykellås = a bike lock
• cykellåsen = the bike lock

In Danish, definiteness is often added as an ending:

• en taske → tasken
• et hjul → hjulet

So cykellåsen ligger i tasken means the bike lock is in the bag.

Danish often uses different verbs for where something is located, depending on how it is positioned.

Here, ligger literally means lies / is lying. It is commonly used for objects that are resting somewhere.

So:

• Cykellåsen ligger i tasken = The bike lock is in the bag

Using er would not be impossible in every context, but ligger sounds more natural because it describes the lock as physically lying in the bag.

Common location verbs in Danish include:

• ligge = lie, be lying
• stå = stand, be standing
• sidde = sit, be attached/sitting somewhere

This is a very common feature of Danish.

Because i means in, while på means on.

• i tasken = in the bag
• på tasken = on the bag

Since the lock is inside the bag, i is the correct preposition.

Den means it and refers back to cykellåsen.

Because cykellås is a common gender noun (en cykellås), the pronoun is den.

Compare:

• en cykellås → den
• et hjul → det

So:

• jeg sætter den altid fast = I always fasten it

This is a very common Danish expression.

• sætte = put / set
• fast = firmly / fixed / attached

Together, sætte noget fast means:

• to fasten something
• to attach something
• to secure something

So:

• jeg sætter den altid fast om hjulet = I always fasten it around the wheel

This is one of those combinations where the full meaning is best learned as a unit.

Because Danish often splits verb combinations in main clauses.

The basic expression is:

• at sætte noget fast

But in a normal main clause, the finite verb comes early, and particles like fast often stay later in the sentence:

• Jeg sætter den fast
• Han sætter cyklen fast
• Vi satte den fast i går

This is similar to separable expressions in some other Germanic languages.

Here om means around.

So:

• om hjulet = around the wheel

This is used because a bike lock is physically put around the wheel.

Examples:

• et tørklæde om halsen = a scarf around the neck
• et reb om træet = a rope around the tree

So the phrase describes how the lock is attached.

Because hjul is a neuter noun in Danish:

• et hjul = a wheel
• hjulet = the wheel

Neuter nouns usually take -et in the definite singular.

Compare:

• en taske → tasken
• et hjul → hjulet

So om hjulet means around the wheel.

This is due to normal Danish sentence structure.

In a main clause, the usual order is roughly:

subject + finite verb + object + adverb + other elements

So:

• jeg = subject
• sætter = finite verb
• den = object
• altid = adverb

That gives:

• jeg sætter den altid fast

A learner may expect always to come earlier because of English, but Danish word order often places adverbs like altid after the object pronoun.

Because men means but, and it introduces a new main clause, not a subordinate clause.

So the second part keeps normal main-clause word order:

• men jeg sætter den altid fast ...

Danish inversion happens when something other than the subject comes first, for example:

• Ved skolen sætter jeg den altid fast

Here Ved skolen is placed first, so the verb must come before the subject:

• Ved skolen sætter jeg ...

But after men, if the subject comes first, there is no inversion:

• men jeg sætter ...

It can mean by the school, near the school, or sometimes at the school, depending on context.

The preposition ved often suggests location beside, near, or by something.

So:

• ved skolen = by/near the school

If the idea is simply that this happens in that area, by the school is often the best translation.

Again, this is the definite form.

• en skole = a school
• skolen = the school

Danish usually marks definiteness with an ending rather than a separate word like the.

So:

• ved skolen = by the school

Yes, in the right context you could.

• cykellåsen is more specific: the bike lock
• låsen just means the lock

If it is already obvious that you are talking about a bike lock, Danish speakers might say låsen. But cykellåsen is clearer and more explicit.

Yes, it sounds natural.

A few especially natural features are:

• ligger for the location of an object
• sætte den fast for fasten/secure it
• om hjulet for around the wheel
• ved skolen for a nearby location

So the sentence feels like normal everyday Danish rather than something translated word-for-word from English.