Hvis jeg drikker kaffe om aftenen, kommer jeg sent i seng.

Yes. Hvis is the normal Danish word for if in a condition.

In this sentence, Hvis jeg drikker kaffe om aftenen ... sets up the condition: under that condition, the result is that I go to bed late.

A useful comparison:

• hvis = if
• når = when

Danish learners often mix these up. In a sentence like this, hvis is natural because the speaker means in the case that I drink coffee in the evening.

Danish often uses the present tense in both parts of a sentence like this when talking about:

• habits
• general truths
• things that regularly happen

So Hvis jeg drikker kaffe om aftenen, kommer jeg sent i seng means something like:

• If I drink coffee in the evening, I go to bed late
• or more naturally in English, If I drink coffee in the evening, I end up going to bed late

This is a general pattern in Danish. You do not need a special future form here.

This is because of the Danish V2 rule: in a main clause, the finite verb usually comes in second position.

The first part of the sentence, Hvis jeg drikker kaffe om aftenen, is a subordinate clause. When that whole clause comes first, the main clause follows, and the finite verb comes before the subject:

• Hvis jeg drikker kaffe om aftenen, kommer jeg sent i seng.

Structure:

• first position: Hvis jeg drikker kaffe om aftenen
• second position: kommer
• then subject: jeg

If the main clause came first, you would get normal subject-verb order:

• Jeg kommer sent i seng, hvis jeg drikker kaffe om aftenen.

Yes, you can. Danish often allows så after an initial if-clause:

• Hvis jeg drikker kaffe om aftenen, så kommer jeg sent i seng.

That is perfectly understandable and natural.

But så is not required, and many sentences leave it out:

• Hvis jeg drikker kaffe om aftenen, kommer jeg sent i seng.

Both are fine. Without så is a little more neutral and streamlined.

Om aftenen means in the evening in a general or habitual sense.

So here it means something like:

• when I drink coffee in the evening
• if I drink coffee in the evenings

By contrast, i aften means this evening / tonight, referring to one specific evening.

Compare:

• Jeg drikker kaffe om aftenen. = I drink coffee in the evening / in the evenings.
• Jeg drikker kaffe i aften. = I am drinking coffee tonight.

So om aftenen is the right choice here because the sentence describes a repeated pattern, not one specific evening.

This is a common time expression in Danish. With om + parts of the day, Danish often uses the definite singular form:

• om morgenen = in the morning
• om eftermiddagen = in the afternoon
• om aftenen = in the evening
• om natten = at night

Even though English often sounds more general, Danish still uses the definite form here. So om aftenen is just the normal idiomatic way to say in the evening.

Literally, yes, kommer is related to come, but in this expression komme i seng is idiomatic. It means:

• get to bed
• end up in bed
• often, in natural English, simply go to bed

So kommer jeg sent i seng means I get to bed late / I go to bed late.

A related expression is gå i seng, which also means go to bed. The difference is roughly:

• gå i seng = the act of going to bed
• komme i seng = getting to bed, often with a sense of result

In this sentence, komme i seng works very well because coffee causes the speaker to end up going to bed late.

Because i seng is a fixed idiomatic expression meaning to bed.

Danish often leaves out the article in certain common expressions involving places or functions:

• gå i skole = go to school
• være på arbejde = be at work
• gå i seng = go to bed
• komme i fængsel = go to prison

So i seng does not mean into a bed in a literal, one-bed-specific way. It means to bed as an activity or state.

If you said i sengen, that would sound more like in the bed, referring to a specific bed.

Sent means late. It is an adverb here, describing when the speaker gets to bed.

In kommer jeg sent i seng, it comes before i seng:

• kommer = finite verb
• jeg = subject
• sent = adverb
• i seng = prepositional phrase / idiomatic expression

That word order is natural in Danish. English also does something similar:

• I go to bed late

Because kaffe here means coffee in general, as a substance. Danish often uses bare nouns like this with uncountable things:

• drikke kaffe = drink coffee
• drikke vand = drink water
• spise brød = eat bread

If you say en kaffe, that usually means:

• a coffee as one serving, especially in a café or everyday speech

So:

• Jeg drikker kaffe om aftenen = I drink coffee in the evening
• Jeg drikker en kaffe om aftenen = I drink a coffee in the evening

Both are possible, but the first is more general.

Yes, the comma here is normal and expected.

It separates the initial subordinate clause from the main clause:

• Hvis jeg drikker kaffe om aftenen, kommer jeg sent i seng.

So the structure is:

1. subordinate clause: Hvis jeg drikker kaffe om aftenen
2. main clause: kommer jeg sent i seng

In written Danish, this comma is standard.

Sometimes you may hear both, but they are not exactly the same.

• hvis = if
• når = when

With hvis, the speaker presents the drinking of coffee as a condition:

• If I drink coffee in the evening, I go to bed late.

With når, it sounds more like something that does happen whenever the situation occurs:

• When I drink coffee in the evening, I go to bed late.

In many real-life situations, both can make sense, but hvis is the clearest match for the English if.