Hon märkte att golvet fortfarande var halt, trots att det inte längre var vått.

Because the sentence is in the past tense.

• märker = present tense, notices
• märkte = past tense, noticed

So Hon märkte att ... means She noticed that ...

The same past-time framing also explains why you later get var instead of är.

They are doing two different jobs.

• att after märkte introduces a content clause: She noticed that ...
• trots att is a fixed conjunction meaning although / even though / despite the fact that

So the structure is:

• Hon märkte att ... = She noticed that ...
• trots att ... = although ...

In other words, the second att is part of trots att, not a separate random extra that.

Because golvet is the definite form: the floor.

• ett golv = a floor
• golvet = the floor

Swedish usually adds the definite article to the end of the noun instead of putting a separate word before it.

So:

• golv = floor / a floor in dictionary form
• golvet = the floor

Because the whole sentence is describing a past situation.

• är = is
• var = was

So:

• golvet fortfarande var halt = the floor was still slippery
• det inte längre var vått = it was no longer wet

Since the main verb märkte is in the past, the subordinate clauses are also in the past here.

fortfarande means still.

In this sentence:

• golvet fortfarande var halt = the floor was still slippery

Its position is normal for Swedish subordinate clauses. After att, Swedish often puts sentence adverbs like fortfarande, inte, alltid, kanske before the finite verb.

So compare:

• Main clause: Golvet var fortfarande halt
• Subordinate clause: ... att golvet fortfarande var halt

That word order is very typical.

trots att means although / even though.

It introduces a contrast:

• the floor was still slippery
• even though it was no longer wet

So the idea is: you might expect a dry floor not to be slippery, but surprisingly it still was.

Common alternatives include:

• fastän = although
• även om = even if / even though, depending on context

But trots att is a very natural choice here.

inte längre means no longer.

So:

• det inte längre var vått = it was no longer wet

A very useful contrast is:

• fortfarande = still
• inte längre = no longer

This sentence nicely uses both:

• fortfarande halt = still slippery
• inte längre vått = no longer wet

Because this is a subordinate clause introduced by att.

In Swedish subordinate clauses, sentence adverbs such as inte, alltid, ofta, fortfarande, and längre typically come before the finite verb.

So:

• Main clause: Det var inte längre vått
• Subordinate clause: ... att det inte längre var vått

This is one of the most important Swedish word-order patterns to learn:

• main clause: verb comes early
• subordinate clause: inte usually comes before the finite verb

Because halt is the neuter form of the adjective hal.

• common gender: hal
• neuter: halt

Since golvet comes from ett golv, it is a neuter noun, so the adjective agrees with it:

• golvet var halt

This is predicative adjective agreement, meaning the adjective comes after a verb like är/var/blir and still agrees with the noun.

A useful pattern:

• en väg är hal = a road is slippery
• ett golv är halt = a floor is slippery

Also note that Swedish halt here does not mean the English verb halt (= stop). It is related to hal, meaning slippery.

Because the base adjective is våt and its neuter form is vått.

• common gender: våt
• neuter: vått

Again, the thing being described is neuter:

• det ... var vått
• or equivalently golvet var vått

Many Swedish adjectives add -t in the neuter, and sometimes spelling changes slightly. Here våt becomes vått with double t.

Compare:

• en handduk är våt = a towel is wet
• ett golv är vått = a floor is wet

Here det refers to the situation/item already being talked about — in practice, the floor.

So the clause means:

• that it was no longer wet
• where it = the floor

Swedish often uses det in this way, just as English uses it.

You could think of the logic as:

• golvet ... trots att det ...
• the floor ... although it ...

Yes, but det is more natural here.

You could theoretically say:

• ... trots att golvet inte längre var vått

That is understandable, but repeating golvet sounds heavier. Using det is smoother and more natural, just like English often prefers it instead of repeating the floor.

The comma separates the main idea from the contrasting subordinate clause.

• main part: Hon märkte att golvet fortfarande var halt
• added contrast: trots att det inte längre var vått

In Swedish, commas before subordinate clauses are somewhat flexible compared with English, and style can vary. In a sentence like this, the comma helps readability and clearly marks the contrast.

So it is natural and correct to write it with a comma here.

Yes — it contains several very useful Swedish patterns:

1. Past tense

   • märkte, var

2. Subordinate clause word order

   • att golvet fortfarande var halt
   • att det inte längre var vått
   • Notice how fortfarande and inte längre come before var

3. Definite noun form

   • golvet = the floor

4. Adjective agreement with neuter nouns

   • halt
   • vått

5. Contrast expression

   • trots att = although / even though

So this is a very good example sentence for grammar as well as vocabulary.