Det tager længe at komme hjem, hvis trafikken står stille.

Here det is a dummy subject, just like it in English sentences such as It takes a long time to get home.

Danish often needs a subject in this kind of sentence, even when det does not refer to a specific thing. So:

• Det tager længe at komme hjem = It takes a long time to get home

You should not try to translate det as a real object here. It is just part of the sentence structure.

Because Danish uses tage in the same way English uses take for duration:

• Det tager længe = It takes a long time
• Det tager fem minutter = It takes five minutes

So tage is the normal verb when you talk about how much time something requires.

Because længe is an adverb, while lang is an adjective.

In this sentence, the word describes how long the action takes, so Danish uses the adverb:

• Det tager længe = It takes a long time

Compare:

• en lang vej = a long road
• længe = for a long time / a long time

A very common alternative is:

• Det tager lang tid at komme hjem

That version is also natural. The sentence you were given uses længe instead.

It is an infinitive phrase meaning to get home or to come home.

After det tager ..., Danish often uses at + infinitive to say what action takes time:

• Det tager længe at lære dansk = It takes a long time to learn Danish
• Det tager fem minutter at gå derhen = It takes five minutes to walk there

So in your sentence:

• at komme hjem = the thing that takes a long time

Because hjem works as an adverb meaning home. Danish often uses it without a preposition.

So:

• komme hjem = get home / come home
• gå hjem = go home
• køre hjem = drive home

This is similar to English, where we also usually say go home, not go to home.

In this sentence, get home is usually the best English equivalent.

• komme hjem can literally look like come home
• but in time/duration sentences, English often prefers get home

So:

• Det tager længe at komme hjem
  usually corresponds to
  It takes a long time to get home

Hvis means if.

It introduces a condition:

• hvis trafikken står stille = if the traffic is at a standstill

So the sentence means that getting home takes a long time under that condition.

A learner may wonder about når instead. Very roughly:

• hvis = if
• når = when

So hvis is correct here because the sentence is talking about a possible situation, not a guaranteed one.

Because hvis trafikken står stille is a subordinate clause, and the normal order here is:

• conjunction + subject + verb

So:

• hvis = if
• trafikken = the traffic
• står = stands / is
• stille = still

That gives:

• hvis trafikken står stille

This is actually quite close to normal English word order.

A useful extra point: if the hvis-clause comes first, the main clause would usually invert:

• Hvis trafikken står stille, tager det længe at komme hjem.

Notice tager det there, not det tager.

Literally, står stille means stands still.

But with traffic, it is an idiomatic way to say:

• the traffic is not moving
• the traffic is at a standstill
• traffic is completely stopped

So:

• trafikken står stille = the traffic is at a standstill

This is a very natural Danish expression.

That is just the normal idiomatic verb in Danish.

Danish often uses stå in situations where something is stationary or not moving:

• Bilen står stille = The car is standing still / not moving
• Toget står stille = The train is standing still
• Trafikken står stille = The traffic is at a standstill

So even though traffic is not a person, stå stille is the standard expression.

The present tense in Danish is often used for general truths, habitual situations, or typical results.

So this sentence means something like:

• In general, it takes a long time to get home if traffic is at a standstill.

It is not limited to what is happening right now. English does the same thing:

• It takes a long time to get home if the traffic is at a standstill.

Yes. That is completely natural and very common.

Both are correct:

• Det tager længe at komme hjem ...
• Det tager lang tid at komme hjem ...

A simple way to think about the difference:

• længe focuses on duration in an adverb-like way
• lang tid names the duration as a noun phrase, literally long time

In everyday Danish, det tager lang tid is probably the more common pattern, but det tager længe is also perfectly good Danish.