Hvis det er varmt, føles en let brise behagelig på terrassen.

This is because Danish is a V2 language: in a main clause, the finite verb normally comes in the second position.

Here, the whole subordinate clause Hvis det er varmt takes the first position. That means the finite verb of the main clause, føles, must come next:

• Hvis det er varmt, føles en let brise behagelig på terrassen.

So even though en let brise is the subject of the main clause, it comes after the verb because the first slot is already occupied.

If you start with the subject instead, the order becomes more like English:

• En let brise føles behagelig på terrassen, hvis det er varmt.

In this sentence, føles means feels in the sense of seems / is experienced as.

So:

• En let brise føles behagelig = A light breeze feels pleasant

The -s form can look passive, but here it is best understood as a very common Danish way of expressing how something feels. It does not translate as a normal English passive.

Compare:

• Jeg føler brisen = I feel the breeze
• Brisen føles behagelig = The breeze feels pleasant

So for a learner, the safest approach is: memorize føles here as feels.

Here det is a dummy subject, just like English it in if it is warm.

It does not refer to a specific thing. Danish usually needs a subject, even in weather and general-condition expressions:

• Det er varmt = It is warm
• Det regner = It is raining
• Det blæser = It is windy

So det here is grammatical, not meaningful in a concrete way.

Because Hvis det er varmt is a subordinate clause placed before the main clause.

Danish normally separates that opening clause from the main clause with a comma:

• Hvis det er varmt, føles en let brise behagelig på terrassen.

So the comma marks the boundary between:

1. the if-clause
2. the main statement

This is very common in Danish.

Brise is a common-gender noun in Danish, so it takes en:

• en brise = a breeze

The adjective let means light here, as in not strong.

So:

• en let brise = a light breeze

A useful thing to know is that let already has the form used here. Even though Danish adjectives often change form, let stays let in this phrase.

Because behagelig is agreeing with en let brise, and brise is a common-gender singular noun.

Predicate adjectives in Danish often agree with the subject:

• En brise er behagelig
• Et rum er behageligt
• To briser er behagelige

So in this sentence:

• en let brise → common gender singular
• therefore: behagelig

If the subject were a neuter noun, you would use behageligt instead.

Because på is the natural preposition for being on / out on a terrace, balcony, patio, and similar surfaces or open spaces.

• på terrassen = on the terrace / out on the terrace

Using i would sound wrong here, because a terrace is not usually thought of as an enclosed space you are in.

This is similar to English, where on the terrace is the natural choice.

In Danish, the definite article is often attached to the end of the noun.

So:

• en terrasse = a terrace
• terrassen = the terrace

This is one of the big differences from English. Instead of putting the before the noun, Danish usually adds -en, -et, or -ne to the noun itself.

Here:

• terrasse
  • -n/-en → terrassen

Yes, but the meaning changes slightly.

• Hvis det er varmt = If it is warm
  This sounds conditional: in the case that it is warm.

• Når det er varmt = When it is warm
  This sounds more like a general recurring situation: whenever it is warm.

So both can work, but they are not identical. In your sentence, hvis presents the warmth more as a condition.

Yes. Danish allows different word orders, but the verb-second rule still applies in the main clause.

Original:

• Hvis det er varmt, føles en let brise behagelig på terrassen.

Also possible:

• En let brise føles behagelig på terrassen, hvis det er varmt.

Both are correct. The difference is mainly focus:

• starting with Hvis det er varmt emphasizes the condition
• starting with En let brise emphasizes the breeze

Just remember: whatever comes first in the main clause, the finite verb still has to be in second position.

No. Here let means light in the sense of gentle / not strong.

Danish let can mean different things depending on context:

• let opgave = easy task
• let brise = light breeze
• let mad = light food

So you have to interpret it from the noun it describes. With brise, the meaning is clearly gentle or light.