Så länge barnen sover, kan vi sitta i vardagsrummet och prata.

Yes. Så länge is a fixed expression meaning as long as in this sentence.

It is important not to read it word by word as so long in English. In Swedish:

• så länge + clause = as long as / while
• Så länge barnen sover = as long as the children are sleeping / as long as the children sleep

You will also see så länge som, which can mean the same thing:

• Så länge (som) barnen sover ...

Barnen is the definite plural form of barn.

• ett barn = a child
• barn = children
• barnen = the children

So Swedish is being specific here: the children, not just children in general.

A useful thing to notice is that barn is one of those nouns whose singular and plural indefinite forms look the same:

• ett barn
• två barn

Then the definite plural adds -en:

• barnen

Because this part is a normal subordinate clause: barnen sover = the children sleep / are sleeping.

In Swedish subordinate clauses, the subject usually comes before the verb in the normal way:

• barnen sover

There is nothing unusual happening in that clause. The unusual-looking word order comes in the main clause after the comma.

This is one of the most important Swedish word-order rules: verb-second (often called V2) in main clauses.

The sentence begins with a fronted subordinate clause:

• Så länge barnen sover, ...

After that, the main clause starts, and in a Swedish main clause the finite verb must come in the second position. So you get:

• Så länge barnen sover, kan vi sitta i vardagsrummet och prata.

Not:

• Så länge barnen sover, vi kan ...

You can compare:

• Vi kan sitta i vardagsrummet och prata.
• I vardagsrummet kan vi sitta och prata.
• Så länge barnen sover, kan vi sitta i vardagsrummet och prata.

In each case, the finite verb kan stays in second position in the main clause.

In modern Swedish, the comma is often optional in a sentence like this, especially before or after subordinate clauses. So both of these can be seen:

• Så länge barnen sover, kan vi sitta i vardagsrummet och prata.
• Så länge barnen sover kan vi sitta i vardagsrummet och prata.

The comma is often used to make the sentence easier to read. So it is natural and correct, but you may also see the sentence without it.

Because Swedish usually uses the definite form when talking about a specific room in a house, especially in everyday situations.

• ett vardagsrum = a living room
• vardagsrummet = the living room

So:

• i vardagsrummet = in the living room

This is very common in Swedish when the place is understood from the situation. English often does the same, but Swedish can be even more regular about using the definite form in these contexts.

Because kan is a modal verb, and modal verbs are followed by the infinitive without att.

So:

• kan sitta = can sit
• kan prata would also work

In this sentence, prata is coordinated with sitta by och:

• kan vi sitta i vardagsrummet och prata

This means:

• kan governs sitta
• prata is linked to sitta with och

You can think of it as:

• can sit in the living room and talk

No att is needed after kan.

Yes, but it would mean something slightly different.

• sitta = sit
• vara = be / stay

So:

• kan vi sitta i vardagsrummet och prata suggests physically sitting there and talking
• kan vi vara i vardagsrummet och prata means more generally being in the living room and talking

Swedish often uses posture verbs like sitta very naturally, sometimes more often than English would use sit. So sitta sounds very normal here.

They are close in meaning, but prata is generally more everyday and conversational.

• prata = talk
• tala = speak, often a bit more formal or neutral

In ordinary speech, prata is very common and natural in a sentence like this.

You could say:

• ... och prata
• ... och tala

But prata sounds more natural in casual everyday Swedish.

Sover is present tense.

But just like in English, the present tense can cover more than one idea depending on context. Here it can naturally be understood as:

• sleep
• are sleeping

So Så länge barnen sover can mean something like:

• as long as the children sleep
• as long as the children are sleeping

Swedish often uses the simple present where English might prefer the progressive.

A rough guide for an English speaker:

• så sounds roughly like soh, but with a Swedish vowel
• länge is roughly LEN-geh, but that is only approximate

A few pronunciation notes:

• å in så is a long vowel, somewhat like the vowel in British law, but not exactly the same
• ä in länge is like an open e sound
• ng in länge is like the ng in sing
• the final e in länge is pronounced, unlike silent e in English

If you want a very rough learner-friendly version:

• så länge ≈ saw LENG-eh

But remember that Swedish vowel sounds do not match English perfectly, so this is only a guide.

Not naturally in this sentence. Och is needed to connect the two actions:

• sitta i vardagsrummet
• prata

So:

• kan vi sitta i vardagsrummet och prata

Without och, the sentence would sound incomplete or wrong.

Swedish often uses och exactly where English uses and to join two infinitives or verb phrases.

Yes. Swedish allows different elements to come first, but then the V2 rule still applies in the main clause.

For example:

• Vi kan sitta i vardagsrummet och prata så länge barnen sover.
• I vardagsrummet kan vi sitta och prata så länge barnen sover.

All of these are grammatical, but the focus changes slightly depending on what comes first.

The version you were given starts with Så länge barnen sover, which emphasizes the condition or time frame first.