Hvis jeg har ondt i maven, drikker jeg varm te.

Because Danish uses the V2 rule in main clauses: the finite verb usually comes in second position.

In this sentence, the first part Hvis jeg har ondt i maven is a subordinate clause placed first. After that, the main clause starts, and its verb must come before the subject:

• Hvis jeg har ondt i maven, drikker jeg varm te.

So:

• first position = the whole hvis-clause
• second position = drikker
• then comes the subject = jeg

If you wrote Hvis jeg har ondt i maven, jeg drikker varm te, that would sound ungrammatical in Danish.

Hvis means if.

It introduces a conditional clause, meaning a clause that gives the condition for the main clause:

• Hvis jeg har ondt i maven = If I have a stomachache / If my stomach hurts
• drikker jeg varm te = I drink warm tea

So hvis is used when one thing happens under a certain condition.

In Danish, at have ondt is a very common way to say to be in pain or to hurt.

So:

• jeg har ondt = I am in pain / it hurts
• jeg har ondt i maven = my stomach hurts / I have a stomachache

About ondt specifically:

• the basic adjective is ond = bad, painful, or sore in some contexts
• ondt is the form used in this fixed expression have ondt

So you should learn have ondt as a chunk.

Danish often uses have ondt i + body part to say where something hurts.

Examples:

• have ondt i hovedet = have a headache
• have ondt i ryggen = have pain in your back
• have ondt i maven = have a stomachache

So i is the normal preposition in this pattern.

Even though English often uses my stomach hurts, Danish prefers this structure with have, ondt, and i.

Maven is the definite form of mave:

• en mave = a stomach
• maven = the stomach

In expressions about body parts, Danish very often uses the definite form:

• i maven
• i hovedet
• i ryggen

This is idiomatic Danish. Even though English often says my stomach, Danish commonly uses the stomach in these expressions.

Because te is usually treated as an uncountable noun here, like tea in English.

So drikker jeg varm te means something like:

• I drink warm tea
• I drink some warm tea

You do not need en here.

If you said en varm te, it would sound more like:

• a warm tea
• one serving / one cup of warm tea

That is possible in some situations, but in this sentence the more natural general statement is varm te with no article.

Because te is a common gender noun in Danish:

• en te

When an adjective comes before a singular indefinite common-gender noun, it usually takes the basic form:

• varm te

Compare:

• en varm kop = a warm cup
• et varmt bad = a warm bath

So:

• common gender (en-word) → varm
• neuter (et-word) → varmt

Since te is an en-word, varm is correct.

Most naturally, it sounds like a general habit or usual reaction:

• If I have a stomachache, I drink warm tea.

Danish present tense often covers this kind of meaning. It can describe:

• a habit
• something generally true
• what someone normally does

So har and drikker are both in the present tense, but the sentence is really about what happens whenever that situation occurs.

Yes, and the difference is useful.

• Hvis = if
• Når = when / whenever

So:

• Hvis jeg har ondt i maven, drikker jeg varm te.
  = If I have a stomachache, I drink warm tea.

• Når jeg har ondt i maven, drikker jeg varm te.
  = When(ever) I have a stomachache, I drink warm tea.

In real life, if the speaker means this is something that regularly happens, når may actually sound very natural.
Hvis keeps the idea more conditional: in the case that this happens.

Because the sentence begins with a subordinate clause:

• Hvis jeg har ondt i maven

and then moves to the main clause:

• drikker jeg varm te

The comma marks the boundary between those two parts. In Danish writing, commas are commonly used to separate this kind of introductory clause from the main clause.

So the comma helps show the structure clearly:

• condition first
• result second