Det er varmt herinde.

In Danish, det is often used as a dummy subject, just like English it in sentences like It is warm or It is raining.

So in Det er varmt herinde, det does not refer to a specific thing. It simply fills the subject position because Danish sentences usually need a subject.

This is very common with:

• weather: Det regner — It’s raining
• temperature: Det er koldt — It’s cold
• general conditions: Det er mørkt — It’s dark

Because the adjective is in the neuter singular form.

The basic adjective is:

• varm — common gender / general dictionary form

But Danish adjectives often add -t in the neuter singular:

• et varmt rum — a warm room

In a sentence like Det er varmt herinde, the adjective is used predicatively (after er), and with impersonal det expressions like this, Danish normally uses the -t form:

• Det er koldt
• Det er varmt
• Det er mørkt

So varmt is the correct form here.

Herinde means in here.

It is built from:

• her — here
• inde — inside / indoors / in

So herinde specifically means here inside, not just here in a general sense.

Compare:

• her — here
• inde — inside
• herinde — in here

In this sentence, herinde tells you that the warmth is inside this place.

Yes, you often can.

• Det er varmt her = It’s warm here
• Det er varmt herinde = It’s warm in here

The version with herinde is more specific about being inside. If you are contrasting indoors with outdoors, herinde is especially natural.

For example:

• Det er koldt derude, men varmt herinde.
  It’s cold out there, but warm in here.

This is the normal Danish main-clause order:

subject + verb + other elements

So:

• Det = subject
• er = verb
• varmt = predicate adjective
• herinde = place expression

This is very similar to English:

• It is warm in here

Danish is a V2 language, which means the finite verb is usually in the second position in main clauses. In this basic sentence, the subject comes first, so the verb naturally comes second:

• Det er ...

Yes, absolutely.

That version is also correct:

• Herinde er det varmt

Here, herinde is moved to the front for emphasis. Because Danish is a V2 language, the verb still has to stay in second position:

• Herinde = first element
• er = second element
• det = then comes the subject

This version feels a bit more like:

• In here, it’s warm

So both are correct, but they emphasize slightly different things.

Usually varmt means warm, but depending on context it can sometimes cover what English might call hot.

Danish does not always divide temperature words in exactly the same way English does. Context matters.

For example:

• Det er varmt herinde usually means It’s warm in here
• In some situations, especially if the room feels unpleasantly warm, English might naturally translate it as It’s hot in here

If Danish really wants to emphasize too hot, speakers might say things like:

• Det er for varmt herinde — It’s too warm / too hot in here

You add ikke after the verb:

• Det er ikke varmt herinde. — It isn’t warm in here.

That placement is very common in Danish main clauses:

• subject + verb + ikke

Examples:

• Det er ikke koldt.
• Det er ikke mørkt herinde.

Er is the present tense of at være — to be.

So:

• jeg er — I am
• du er — you are
• han/hun/det er — he/she/it is
• vi er — we are

In this sentence, er means is:

• Det er varmt herinde — It is warm in here

Inde means inside / indoors, while herinde means in here.

So:

• inde = inside
• herinde = in here
• derinde = in there

Examples:

• Jeg er inde. — I am inside.
• Jeg er herinde. — I am in here.
• Han er derinde. — He is in there.

In your sentence, herinde points to the speaker’s current indoor location.

You invert the subject and the verb:

• Er det varmt herinde? — Is it warm in here?

This is the normal way to form a yes/no question in Danish:

• statement: Det er varmt herinde.
• question: Er det varmt herinde?

That pattern is very common:

• Det er koldt. → Er det koldt?
• Det er mørkt herinde. → Er det mørkt herinde?

A careful learner-friendly approximation is:

• varmt ≈ vahmt
• herinde ≈ heh-RIN-de

A few important notes:

• In varmt, the written r is not pronounced like a strong English r
• The consonant cluster can sound a bit compressed in normal speech
• In herinde, the stress is usually on the second part: -rin-

As always, the best way to learn this is by listening to native pronunciation a few times and repeating it aloud.