Pludselig ringer telefonen, mens jeg læser, og så lægger jeg bogen fra mig.

Because Danish main clauses normally follow the V2 rule: the finite verb must come in the second position.

Here, Pludselig takes the first position, so the verb ringer has to come next:

• Pludselig ringer telefonen
  = first element + verb + subject

If the sentence started with the subject instead, you would get:

• Telefonen ringer pludselig

So the unusual-looking order is actually very normal Danish word order.

Pludselig is an adverb meaning suddenly.

It sets the scene and tells you that the phone call happens unexpectedly. Putting it first gives it extra emphasis, a bit like English Suddenly, the phone rings...

It also triggers the word order change in the main clause because of the V2 rule.

Mens means while.

The clause mens jeg læser is a subordinate clause, and subordinate clauses in Danish usually do not follow the V2 rule. Instead, they usually keep the more straightforward order:

• mens jeg læser
• conjunction + subject + verb

So:

• mens jeg læser = while I am reading

That is why you do not get mens læser jeg.

Danish usually uses the simple present where English often uses the present progressive.

So:

• jeg læser can mean I read or I am reading, depending on context.

In this sentence, because the action is happening at that moment, jeg læser is naturally understood as I am reading.

This is very common in Danish.

That -en is the definite article attached to the noun.

In English, you say:

• the phone
• the book

In Danish, instead of a separate word like the, you often add the definite ending to the noun:

• telefon = phone

• telefonen = the phone

• bog = book
• bogen = the book

Both telefon and bog are common-gender nouns, so they take -en in the definite singular.

Because after og, the next clause is still a main clause, and main clauses still follow the V2 rule.

In og så lægger jeg bogen fra mig:

• så takes the first position in the clause
• lægger must come second
• jeg comes after the verb

So the pattern is:

• og + så + lægger + jeg ...

Not:

• og så jeg lægger ...

That would sound wrong in standard Danish.

Here så is a linking adverb meaning something like then, so, or and then.

It helps move the story forward:

• the phone rings
• then I put the book down

In this sentence, så has a natural narrative function. It does not mean so in the sense of therefore as strongly as English sometimes does; it is more like then / and then.

It means I put the book down or I lay the book aside/down.

This is built from:

• lægger = put/lay
• bogen = the book
• fra mig = away from me / from my hands

So the whole expression lægge noget fra sig means to put something down.

With jeg, that becomes:

• lægger bogen fra mig

It is a very natural Danish expression.

Because the phrase is really lægge noget fra sig = put something down.

The last part is a pronoun that changes according to the subject:

• jeg lægger bogen fra mig
• du lægger bogen fra dig
• han lægger bogen fra sig

So mig is used because the subject is jeg.

Without that pronoun, the meaning would feel incomplete or different.

Because sig is the reflexive form used for third person subjects.

Compare:

• Jeg lægger bogen fra mig = I put the book down
• Han lægger bogen fra sig = He puts the book down

Since the subject here is jeg, the correct pronoun is mig, not sig.

Danish often chooses different placement verbs depending on how something is positioned.

A book is thought of as something that is typically placed lying flat, so lægge is natural.

Very roughly:

• lægge = lay/put something so it lies
• stille = put something so it stands
• sætte = put something so it sits / place it seated

So with bog/book, lægge is the expected choice.

The commas help separate the clauses:

• Pludselig ringer telefonen
• mens jeg læser
• og så lægger jeg bogen fra mig

The middle part, mens jeg læser, is a subordinate clause inserted between two main-clause parts, so commas make the structure clearer.

Danish comma rules can be a little tricky, and you may see slight variation depending on comma style, but in a sentence like this, the commas help show where the inserted mens-clause begins and ends.

Yes, that is grammatically possible, but it is not exactly the same in emphasis.

• Pludselig ringer telefonen puts focus on the suddenness first.
• Telefonen ringer pludselig starts with the phone and sounds less dramatic.

The original version is more natural in storytelling because it creates the feeling of an interruption.

No, that would not sound right.

If you want a normal main clause after og, you would usually say either:

• ..., og så lægger jeg bogen fra mig
• ..., og jeg lægger bogen fra mig

Both are possible, but they mean slightly different things in rhythm and emphasis.

• og så lægger jeg ... = and then I put ...
• og jeg lægger ... = and I put ...

In your sentence, så makes the sequence of events feel especially natural.