Hvis jeg kommer hjem sent, drikker jeg kun vand og går i seng.

Because Danish has verb-second (V2) word order in main clauses.

When the sentence starts with something other than the subject, the finite verb comes before the subject. Here, the sentence begins with the if-clause:

Hvis jeg kommer hjem sent, ...

After that clause, the main clause starts, and Danish puts the verb first:

drikker jeg kun vand

So the pattern is:

• neutral main clause: Jeg drikker kun vand.
• after an opening clause: Hvis ..., drikker jeg kun vand.

This is very common in Danish.

Because hvis introduces a subordinate clause, and subordinate clauses normally keep the basic order:

subject + verb + other elements

So:

• hvis jeg kommer hjem sent = correct
• hvis kommer jeg hjem sent = incorrect

In other words:

• main clause: often V2
• subordinate clause: usually normal subject-verb order

Danish often uses the present tense to talk about the future, especially when the meaning is clear from context.

So Hvis jeg kommer hjem sent can mean:

• If I come home late
• If I get home late

Likewise, drikker jeg and går jeg are present forms, but they can describe what the speaker usually does or what they will do in that situation.

English does something similar in if-clauses:

• If I come home late, I only drink water and go to bed.

Not if I will come.

Hjem means home, but in Danish it often works as an adverb of direction, not just a noun.

So:

• jeg kommer hjem = I come home / I get home

You do not need an article like to the home or to home.

Compare:

• Jeg er hjemme = I am at home
• Jeg kommer hjem = I come home / I get home

This difference is very important in Danish:

• hjem = movement toward home
• hjemme = location at home

Because sent here is an adverb, meaning late.

Adverbs do not agree for gender or number the way adjectives can. It describes when or how the action happens:

• kommer hjem sent = come home late

So sent is fixed here.

Compare:

• adjective: en sen aften = a late evening
• adverb: jeg kommer sent = I arrive late

Kun means only, and its position shows what it focuses on.

Here:

drikker jeg kun vand

means that the speaker drinks only water.

So kun is placed right before the thing being limited: vand.

Compare the emphasis:

• Jeg drikker kun vand = I drink only water
• Jeg kun drikker vand = unnatural/incorrect in standard Danish word order here

Word placement matters a lot with words like kun, because it changes what the sentence emphasizes.

Because Danish, like English, can leave out the subject in the second part of a coordinated structure when it is the same subject.

So:

drikker jeg kun vand og går i seng

means:

I only drink water and go to bed

The full version would be:

drikker jeg kun vand, og jeg går i seng

But repeating jeg is unnecessary here and sounds less natural.

Går i seng literally means goes into bed, but idiomatically it means goes to bed.

It does not necessarily mean the person immediately falls asleep. It focuses on the action of going to bed.

Compare:

• Jeg går i seng = I go to bed
• Jeg sover = I sleep / I am sleeping

So går i seng is the natural phrase if you want to say someone goes to bed.

Because Danish normally places a comma between the subordinate clause and the main clause.

So:

Hvis jeg kommer hjem sent, drikker jeg kun vand og går i seng.

The comma marks the end of the if-clause and the beginning of the main clause.

This is standard Danish punctuation.

Sometimes, but the meaning changes.

• hvis = if
• når = when

So:

• Hvis jeg kommer hjem sent... = If I come home late...
  This is conditional; it may or may not happen.

• Når jeg kommer hjem sent... = When I come home late...
  This suggests it does happen, at least on some occasions.

Learners often confuse these because English sometimes uses when and if loosely, but Danish keeps the distinction clearer.

It can be understood either way, depending on context.

By itself, the sentence most naturally sounds like a habitual or general statement:

If I come home late, I only drink water and go to bed.

That is, this is what the speaker generally does in that situation.

But in the right context, it could also refer to a future situation. Danish present tense is flexible that way.

No. With home in this sense, Danish uses simply hjem without a preposition.

So:

• jeg kommer hjem = correct
• jeg kommer til hjem = incorrect

This is one of those patterns you should learn as a fixed expression:

• tage hjem = go home
• komme hjem = come home / get home
• være hjemme = be at home

Because this is still part of a finite clause with the same subject.

The first finite verb in the main clause is drikker, and går is coordinated with it:

• drikker ... og går ...

Both are present-tense finite verbs:

• drikker
• går

This is parallel to English:

• I drink only water and go to bed

Not:

• I drink only water and to go to bed