Die Schaukel im Park ist heute noch nass vom Regen.

Because Schaukel is a feminine noun in German. Its basic dictionary form is die Schaukel.

German nouns have grammatical gender:

• der = masculine
• die = feminine
• das = neuter

So when you talk about the swing, you say die Schaukel.

Here, Schaukel means swing, as in playground equipment or a swing in a park.

So Die Schaukel im Park means The swing in the park.

Im is just the common contraction of in dem.

• in dem Park = in the park
• im Park = in the park

German uses this contraction very often, and im Park sounds more natural in everyday language.

Also, dem is in the dative case, because in is expressing location here, not movement.

• im Park = located in the park
• in den Park = into the park

The preposition in can take either accusative or dative, depending on meaning.

Use dative when talking about where something is:

• Die Schaukel ist im Park. = The swing is in the park.

Use accusative when talking about movement toward somewhere:

• Ich gehe in den Park. = I am going into the park.

Since the swing is simply located there, German uses the dative: im Park.

Because German main clauses follow the verb-second rule.

In a normal statement, the finite verb comes in the second position:

• Die Schaukel = position 1
• ist = position 2

Everything else comes after that:

• heute noch nass vom Regen

So the structure is completely normal for German:

• Die Schaukel | ist | heute noch nass vom Regen.

Here, heute noch means something like still today or still now, today.

The important word is noch, which often means still.

So:

• heute = today
• noch = still

Together, heute noch nass means still wet today or still wet even now today.

It suggests that the swing has not dried yet.

Not exactly. In this sentence, noch means still.

• Die Schaukel ist noch nass. = The swing is still wet.

If you wanted to say not yet, German usually uses noch nicht:

• Die Schaukel ist noch nicht trocken. = The swing is not dry yet.

So:

• noch = still
• noch nicht = not yet

Because nass is being used as a predicate adjective, after the verb sein.

In German:

• Attributive adjective: before a noun, usually with an ending
  
  • die nasse Schaukel = the wet swing
• Predicate adjective: after sein, werden, bleiben, etc., with no ending
  
  • Die Schaukel ist nass. = The swing is wet.

So nass stays plain here because it comes after ist.

Vom Regen means from the rain.

It explains the cause of the wetness:

• nass vom Regen = wet from the rain

This is a very natural German expression.

Vom is the contraction of von dem.

• von dem Regen → vom Regen

Just like im for in dem, German very commonly contracts von dem to vom.

Also, von takes the dative case, so:

• der Regen becomes dem Regen after von
• therefore: vom Regen

Because the preposition von always takes the dative case.

The noun is der Regen in the nominative:

• der Regen

After von, it becomes dative:

• von dem Regen
• contracted: vom Regen

So the article changes because of the preposition.

Yes, but they mean slightly different things.

• nass vom Regen = wet from the rain
  This is the most natural phrasing here.
• nass durch den Regen = wet because of the rain / through the rain
  Possible, but less idiomatic in this sentence.
• wegen des Regens = because of the rain
  This talks about the reason in a more abstract way, not specifically the wetness source.

So if you want to say the swing is physically wet because rain got on it, nass vom Regen is the best choice.

German often places time expressions fairly early in the sentence, after the verb if the subject comes first.

So this order is very natural:

• Die Schaukel ist heute noch nass vom Regen.

A rough breakdown is:

• subject: Die Schaukel
• verb: ist
• time: heute
• adverb: noch
• adjective: nass
• prepositional phrase: vom Regen

Other word orders can be possible in German, but this one sounds very normal.

That would usually sound different in meaning.

• heute noch = still today / still wet today
• noch heute often means something more like as early as today or even today

So in your sentence, heute noch is the correct and natural choice if the idea is that the swing still hasn't dried.

Then nass would come before the noun and would need an adjective ending:

• die nasse Schaukel im Park = the wet swing in the park

Compare:

• Die Schaukel im Park ist nass. = The swing in the park is wet.
• Die nasse Schaukel im Park = The wet swing in the park

This is a very important difference in German:

• adjective before the noun → adjective ending needed
• adjective after sein → no ending

Yes. It sounds completely natural.

A native speaker would understand it as:

• The swing in the park is still wet today from the rain.

It is a normal, clear sentence using common German patterns:

• noun gender: die Schaukel
• location with dative: im Park
• verb-second word order: ist
• predicate adjective: nass
• common contraction: vom Regen