Hennes regnbyxor hänger i trapphuset och är fortfarande våta efter promenaden.

Because sin/sitt/sina is a reflexive possessive, used when the possessor is the subject of the clause.

In this sentence, the subject is hennes regnbyxor — her rain trousers / rain pants. The owner is she, but she is not the grammatical subject here; the trousers are.

So Swedish uses hennes = her.

• Hennes regnbyxor hänger... = Her rain pants are hanging...
• Hon hänger sina regnbyxor... = She is hanging her rain pants...

In the second example, hon is the subject, so sina works.

Byxor means trousers/pants, and in Swedish it is normally treated as a plural word, just like in English.

So:

• byxor = trousers / pants
• regnbyxor = rain trousers / rain pants

Swedish often uses this plural form even when referring to one item of clothing. If you want to be extra specific, you can say:

• ett par regnbyxor = a pair of rain pants

Because regnbyxor is plural, that also affects the adjective later in the sentence: våta.

Hänger is the present tense of hänga, meaning hang.

Here it means that the rain pants are hanging somewhere, probably drying.

Swedish often uses the simple present where English might prefer is/are hanging:

• Byxorna hänger i trapphuset. = The trousers are hanging in the stairwell.

So hänger is not strange here at all; it is the normal way to describe something being in a hanging position.

The preposition i means in, and trapphuset means the stairwell / the apartment building hallway / the staircase area.

Swedish uses i because the idea is that the pants are inside that space.

So:

• i trapphuset = in the stairwell / in the hallway of the building

This may feel slightly different from English, where people might say in the stairwell, in the hallway, or sometimes out in the hallway, depending on context.

Prepositions often do not match perfectly between Swedish and English, so this is something you mostly learn by exposure.

Trapphuset is the definite form of trapphus:

• ett trapphus = a stairwell
• trapphuset = the stairwell

The definite form is used because it refers to a specific stairwell, probably the one in the building where the person lives.

So the sentence is not talking about some stairwell somewhere, but the stairwell relevant in the situation.

Swedish, like English, often leaves out the subject in the second part of a coordinated sentence when it is the same as before.

So:

• Hennes regnbyxor hänger i trapphuset och är fortfarande våta...

means:

• Her rain pants are hanging in the stairwell and are still wet...

The subject of both verbs is the same: hennes regnbyxor.

You could repeat it, but it would usually sound less natural:

• Hennes regnbyxor hänger i trapphuset och de är fortfarande våta...

That is possible, but the original version is smoother and more natural.

Fortfarande means still.

In this sentence it modifies the idea of being wet, so it comes after är and before the adjective:

• är fortfarande våta = are still wet

This is normal Swedish word order.

Compare:

• De är fortfarande våta. = They are still wet.
• De hänger fortfarande där. = They are still hanging there.

So the position of fortfarande depends on what structure it is in, but in this sentence är fortfarande våta is exactly what you would expect.

Because våta is the plural form of the adjective våt = wet.

Since regnbyxor is plural, the adjective must agree with it:

• våt = wet, singular common gender
• vått = wet, singular neuter
• våta = wet, plural

So:

• en jacka är våt = a jacket is wet
• ett täcke är vått = a blanket is wet
• byxorna är våta = the trousers are wet

Here, regnbyxor is plural, so våta is required.

Efter promenaden means after the walk, referring to a specific walk that is known from context.

• efter promenaden = after the walk
• efter en promenad = after a walk

In this sentence, the speaker probably has one particular walk in mind — for example, the walk they just took. That is why the definite form promenaden sounds natural.

Promenaden is not the subject.

The phrase efter promenaden is a prepositional phrase acting as an adverbial, giving extra information about when/after what event the pants are still wet.

The subject is the whole noun phrase:

• Hennes regnbyxor

Then the rest says something about them:

• hänger i trapphuset
• och är fortfarande våta efter promenaden

So the sentence structure is basically:

• Subject: hennes regnbyxor
• Verb phrase 1: hänger i trapphuset
• Verb phrase 2: är fortfarande våta efter promenaden

Usually regnbyxor means rain pants / waterproof overtrousers — trousers worn to protect against rain.

Literally it is:

• regn = rain
• byxor = trousers/pants

Depending on context, English translations could be:

• rain pants
• waterproof trousers
• rain trousers

So the exact wording in English may vary, but the Swedish word is perfectly ordinary.

Våta simply means wet. It does not by itself tell you exactly how wet.

So it could mean:

• a bit damp
• fairly wet
• very wet

The context decides. In this sentence, since the pants are hanging after a walk, the natural assumption is that they got wet from the weather and are drying.

Yes, it sounds natural.

It is a normal everyday sentence with straightforward Swedish structure:

• Hennes regnbyxor = subject
• hänger i trapphuset = first statement
• och är fortfarande våta = second statement about the same subject
• efter promenaden = extra context

A native speaker would understand it immediately, and nothing about it sounds unnatural or overly formal.