Dansgólfið var tómt þegar vindurinn varð hvass.

Icelandic very often forms compound nouns as a single word.

• dans = dance
• gólf = floor
• dansgólf = dance floor

Then the definite article is added to the end:

• dansgólf = a dance floor
• dansgólfið = the dance floor

So dansgólfið is literally dance-floor-the.

In Icelandic, the is usually attached to the noun instead of standing as a separate word.

In this sentence:

• dansgólfið = dansgólf
  • -ið = the dance floor
• vindurinn = vindur
  • -inn = the wind

This is one of the big differences from English. Icelandic can use a separate demonstrative in some contexts, but the normal definite article is usually a suffix.

Because tómt has to agree with dansgólfið.

• dansgólf is a neuter noun
• dansgólfið is neuter singular
• the adjective must match that

So the adjective tómur changes like this:

• masculine: tómur
• feminine: tóm
• neuter: tómt

Since dansgólfið is neuter singular, tómt is the correct form.

Because hvass has to agree with vindurinn.

• vindur is a masculine noun
• vindurinn is masculine singular
• so the adjective must also be masculine singular

The forms are:

• masculine: hvass
• feminine: hvöss
• neuter: hvasst

So:

• vindurinn varð hvass = correct
• vindurinn varð hvasst = wrong, because hvasst is neuter

They come from two different verbs:

• var = past tense of vera = was
• varð = past tense of verða = became

So the sentence is contrasting:

• var tómt = was empty (a state)
• varð hvass = became strong/harsh (a change)

That distinction is very important in Icelandic.

Yes, but the meaning changes.

• vindurinn var hvass = the wind was strong
• vindurinn varð hvass = the wind became strong

So var describes a condition, while varð describes a change into that condition.

Here þegar means when and introduces a time clause:

• Dansgólfið var tómt þegar vindurinn varð hvass.
• The dance floor was empty when the wind became strong.

Yes, þegar can also mean already in other sentences, but not here. In this sentence, its job is clearly to connect the two clauses in time.

Both are in the nominative, because both are subjects of their clauses.

• Dansgólfið = subject of var tómt
• vindurinn = subject of varð hvass

The predicate adjectives tómt and hvass also appear in nominative forms, agreeing with their subjects in gender and number.

Yes. The structure is:

• main clause: Dansgólfið var tómt
• subordinate clause: þegar vindurinn varð hvass

This is perfectly normal Icelandic word order.

You can also reverse the order:

• Þegar vindurinn varð hvass, var dansgólfið tómt.

That also means the same thing, but when the þegar clause comes first, the main clause usually shows the normal Icelandic verb-second pattern: var dansgólfið.

Yes, hvass can mean sharp, but with weather and wind it often means something like:

• strong
• keen
• harsh
• biting

So vindurinn varð hvass is a natural Icelandic way to say that the wind became strong or cutting.

After verbs like vera and verða, Icelandic often uses a predicate adjective that describes the subject.

Here:

• dansgólfið → tómt
• vindurinn → hvass

So the pattern is:

• subject + vera/verða
  • adjective

And the adjective still has to agree with the subject in gender and number, even though it comes after the verb. That is why you get:

• dansgólfið var tómt
• vindurinn varð hvass