Hennes mobil låg ingenstans på bordet, så hon gick tillbaka till bilen.

Because sin/sitt/sina is only used when the possessor is the same as the subject of that clause.

In Hennes mobil låg ingenstans på bordet, the subject is mobil (phone), not hon (she). So Swedish uses hennes.

Compare:

• Hon tog sin mobil. = She took her own phone.
  
  • Here the subject is hon, so sin works.
• Hennes mobil låg på bordet. = Her phone was on the table.
  
  • Here the subject is mobil, so hennes is correct.

Yes. In everyday Swedish, mobil commonly means mobile phone / cell phone.

It is a shortened form of mobiltelefon, but people usually just say mobil.

So:

• en mobil = a mobile phone
• min mobil = my phone

Swedish often prefers a position verb where English would simply use to be.

Here, låg is the past tense of ligga, which often means:

• to lie
• to be lying
• to be located, especially for things resting horizontally

So mobilen låg på bordet is more natural Swedish than using var.

Very roughly:

• Boken låg på bordet. = The book was lying on the table.
• Nycklarna låg i väskan. = The keys were in the bag.

English often just says was, but Swedish likes to specify position.

Ingenstans means nowhere or not anywhere.

In this sentence, låg ingenstans på bordet means the phone was nowhere on the table / not anywhere on the table.

A useful thing to notice: ingenstans already has a negative meaning, so you do not add inte.

• Den fanns ingenstans. = It was nowhere to be found.
• Jag hittar den ingenstans. = I can’t find it anywhere.

Because ingenstans functions as an adverb, and the location phrase på bordet adds more detail.

So the structure is roughly:

• låg = was lying
• ingenstans = nowhere
• på bordet = on the table

Together: was nowhere on the table

You could think of på bordet as specifying the area in which it was nowhere to be found.

Bordet is the definite form of bord, meaning the table.

• ett bord = a table
• bordet = the table

So:

• på bordet = on the table

Also note that bord is a neuter noun (ett word), which is why the definite ending is -et.

Here så means so, therefore, or as a result.

It connects the two parts of the sentence:

• Her phone wasn’t on the table,
• so she went back to the car.

This is a very common use of så in Swedish.

Because så here is a coordinating conjunction, similar to English so.

After a coordinating conjunction, Swedish keeps normal main-clause word order:

• så hon gick tillbaka till bilen

Subject first, then verb:

• hon gick

If it were a sentence adverb like därför (therefore), inversion would often happen:

• ... därför gick hon tillbaka till bilen.

So this is a useful contrast:

• ..., så hon gick tillbaka ...
• ...; därför gick hon tillbaka ...

Gick tillbaka means went back or walked back.

It is:

• gick = past tense of gå (go / walk)
• tillbaka = back

Together: went back

Depending on context, English may translate it as either:

• went back
• walked back

Since the destination is bilen (the car), the idea is that she returned to it.

Yes, but the meaning would be a bit different.

• gick tillbaka suggests she went back on foot
• åkte tillbaka suggests she went back by vehicle / transport

Since she is going back to the car, gick tillbaka is very natural: she likely walked back to where the car was parked.

Because till means to, showing direction toward a destination.

• till bilen = to the car

And again, bilen is the definite form:

• en bil = a car
• bilen = the car

So hon gick tillbaka till bilen means she went back to the car.

Yes, both are irregular and very common.

Their infinitives are:

• ligga → låg
• gå → gick

These are important verbs to learn early because they appear all the time in everyday Swedish.

It is acceptable and natural here, because the sentence joins two main clauses:

• Hennes mobil låg ingenstans på bordet
• så hon gick tillbaka till bilen

The comma helps separate the cause and result. In Swedish punctuation, commas before conjunctions like så can appear in this kind of longer sentence, especially when it improves clarity.