Hennes handduk ligger kvar i omklädningsrummet, så vi måste gå tillbaka och hämta den.

In Swedish, sin/sitt/sina is a reflexive possessive, used when the owner is the subject of the same clause.

Here, the subject is handduk in the first clause:

• Hennes handduk ligger kvar ...
• literally: Her towel is still left ...

The towel belongs to some woman/girl, but that owner is not the subject of the clause. So Swedish uses hennes, not sin.

Compare:

• Hon hämtar sin handduk. = She fetches her own towel.
• Hon hämtar hennes handduk. = She fetches her/some other woman’s towel.

So in your sentence, hennes is the correct choice.

In Swedish, when you use a possessive like min, din, hans, hennes, vår, you normally do not add the definite ending.

So:

• hennes handduk = her towel
  not
• hennes handduken

This is different from English, where the and possessives are also not used together in normal phrases: you say her towel, not her the towel.

A few examples:

• min bok = my book
• deras bil = their car
• vårt hus = our house

So hennes handduk is the normal Swedish pattern.

Swedish often uses position verbs where English would simply use is or has been left.

Here, ligger literally means lies/is lying, but in natural English the idea is more like:

• is left
• is still there
• is lying there

Swedish commonly chooses among verbs like:

• ligga = lie, be lying
• stå = stand, be standing
• sitta = sit, be sitting

The choice often depends on how something is positioned or conceived.

For example:

• Boken ligger på bordet. = The book is on the table.
• Flaskan står på bordet. = The bottle is on the table.
• Tavlan sitter på väggen. = The picture is on the wall.

So handduk ligger sounds very natural in Swedish.

Kvar means something like:

• left behind
• still there
• remaining

So ligger kvar means that the towel is still there / has been left behind.

A few examples:

• Jag är kvar hemma. = I’m still at home.
• Det finns lite kaffe kvar. = There is a little coffee left.
• Hon glömde boken, så den låg kvar på bussen. = She forgot the book, so it was left on the bus.

In your sentence, ligger kvar strongly suggests the towel was accidentally left in the changing room.

It’s a compound noun:

• omklädning = changing, getting changed
• rum = room
• -et = the definite ending for a neuter noun

So:

• ett omklädningsrum = a changing room
• omklädningsrummet = the changing room

Swedish makes a lot of compound nouns this way. English often writes them as two words, but Swedish frequently combines them into one.

Because the meaning is in the changing room, not on the changing room.

Swedish uses i for being inside an enclosed space:

• i huset = in the house
• i bilen = in the car
• i omklädningsrummet = in the changing room

På is used in other cases, for example:

• on a surface: på bordet = on the table
• at certain places/institutions by idiom: på jobbet = at work, på sjukhuset = at the hospital

But for a room like this, i is the natural choice.

Here så means so / therefore.

It connects the first clause to the result:

• Hennes handduk ligger kvar i omklädningsrummet, så vi måste gå tillbaka ...
• Her towel is left in the changing room, so we have to go back ...

In many contexts, Swedish så does match English so, but it has other uses too, such as:

• så stor = so big
• så! = there!/like that!
• Han sa så. = He said so.

In this sentence, it is a conjunction meaning so.

Because måste is a modal verb, and modal verbs in Swedish are followed by the bare infinitive.

So:

• måste gå
• kan komma
• vill äta
• ska läsa

not:

• måste att gå

This is similar to English:

• must go
• can come
• will read

not:

• must to go

So vi måste gå tillbaka och hämta den is completely normal Swedish.

Gå tillbaka means go back or return.

It is made of:

• gå = go, walk
• tillbaka = back

Depending on context, it can mean physically go back to a place, or sometimes go back in a more abstract sense.

Examples:

• Vi måste gå tillbaka till bilen. = We have to go back to the car.
• Kan du gå tillbaka en sida? = Can you go back one page?

In your sentence, it means they need to return to the changing room.

Den refers back to handduk.

Swedish nouns have grammatical gender, and handduk is an en-word:

• en handduk = a towel

For a singular common-gender noun, the object pronoun is den:

• Jag ser boken. Jag tar den.
• Vi glömde handduken. Vi hämtar den.

If the noun were a t-word (ett word), the pronoun would usually be det:

• ett äpple → det
• ett rum → det

So den is correct because it refers to en handduk.

Using den avoids unnecessary repetition, just like English it.

So Swedish prefers:

• ... gå tillbaka och hämta den. = ... go back and get it.

instead of:

• ... gå tillbaka och hämta handduken.

Both are possible, but den sounds more natural when the referent is already obvious.

Hämta means fetch, go and get, or pick up. It often implies going to a place and bringing something back.

Ta just means take, which is broader.

So in this sentence:

• gå tillbaka och hämta den = go back and fetch it

That works well because they need to return to the changing room to get the towel.

Compare:

• Jag ska hämta min jacka. = I’m going to get/fetch my jacket.
• Ta en kaka. = Take a cookie.

So hämta is the better verb here.

Yes, så vi måste is normal here.

That så is acting as a coordinating conjunction, linking two main clauses:

• Hennes handduk ligger kvar i omklädningsrummet
• så vi måste gå tillbaka och hämta den

In coordinated main clauses, Swedish generally keeps normal main-clause word order:

• vi måste

If you put another element first inside the clause, then inversion can happen:

• Därför måste vi gå tillbaka. = Therefore we must go back.

So:

• ..., så vi måste gå tillbaka ... = natural
• ..., därför måste vi gå tillbaka ... = also natural, but slightly different structure

Sometimes yes, but the meaning changes slightly.

• är kvar = is left / remains / is still there
• ligger kvar = is still lying there / has been left there

Because a towel is a physical object lying somewhere, ligger kvar sounds vivid and natural.

Compare:

• Min väska är kvar på tåget. = My bag is still on the train.
• Min väska ligger kvar på tåget. = My bag is still lying on the train.

Both can work, but ligger kvar gives a stronger sense of the object’s physical presence in that place.