Min cykellås er tung, men den er nødvendig, hvis jeg parkerer tæt på stationen.

Because cykellås is a common gender noun in Danish.

• en cykellås = a bike lock
• so the possessive is min = my

Danish possessives change with the gender and number of the noun:

• min for common gender singular
• mit for neuter singular
• mine for plural

So:

• min cykellås = my bike lock
• mit hus = my house
• mine cykler = my bikes

Because Danish usually forms compound nouns by joining them into a single word.

Here:

• cykel = bike
• lås = lock
• cykellås = bike lock

This is very common in Danish, much more than in English. The last part is the main noun, and the first part describes it.

Other examples:

• togstation = train station
• cykelhjelm = bike helmet
• sommerferie = summer holiday

Den refers back to min cykellås.

Since cykellås is a common gender noun, the pronoun is den:

• cykellåsen → den
• a neuter noun would take det

So:

• Min cykellås er tung, men den er nødvendig
• Mit tæppe er tungt, men det er nødvendigt would use det if the noun were neuter

In English, we just use it for both.

Because the adjective agrees with the noun it describes.

Here, cykellås is:

• singular
• common gender

So the adjective stays in its basic form:

• tung

Compare:

• en cykellås er tung
• et skab er tungt
• cykellåse er tunge

This is happening after er, so the adjective is being used as a predicate adjective.

For the same reason as tung: it agrees with den, which refers to cykellås, a singular common gender noun.

So:

• den er nødvendig = it is necessary

If the noun were neuter, you would use nødvendigt:

• det er nødvendigt

And with plural:

• de er nødvendige

Hvis means if.

It introduces a condition:

• hvis jeg parkerer tæt på stationen = if I park close to the station

So the sentence means that the heavy lock is necessary in that situation.

This is a normal real condition, just like English:

• If I park close to the station, I need it.

Yes. Hvis starts a subordinate clause.

In this example, the word order looks simple:

• hvis jeg parkerer tæt på stationen

But subordinate clauses in Danish do not follow the same main-clause pattern as ordinary statements. A useful way to see this is with ikke:

• main clause: Jeg parkerer ikke tæt på stationen
• subordinate clause: hvis jeg ikke parkerer tæt på stationen

So after hvis, words like ikke usually come before the finite verb, not after it.

Tæt på is a fixed expression meaning close to.

• tæt = close / tight
• på = on / at / to, depending on context

Together, tæt på means close to:

• tæt på huset = close to the house
• tæt på stationen = close to the station

So you should learn tæt på as a set phrase.

Stationen is the definite form: the station.

Danish usually adds the definite article to the end of the noun:

• en station = a station
• stationen = the station

Here, it suggests a specific station that is known from the context, probably the local station or the one the speaker has in mind.

The comma before men is standard, because men joins two full clauses:

• Min cykellås er tung
• den er nødvendig

The comma before hvis reflects Danish comma style. Many writers put a comma before a subordinate clause like this:

• ..., hvis jeg parkerer tæt på stationen

In modern Danish, you may also see this written without that comma, depending on the comma system being used. So the comma before hvis is common and acceptable, but it is not the same kind of fixed requirement as the comma before men.

Yes. You could also say:

• Hvis jeg parkerer tæt på stationen, er min cykellås nødvendig.

Or more naturally:

• Hvis jeg parkerer tæt på stationen, er den nødvendig.

When the hvis-clause comes first, the main clause follows it. Danish main clauses still keep the finite verb early in the clause, so you get:

• Hvis ..., er den ...

not

• Hvis ..., den er ...