Learn Hardcore Swedish: När de åker till festen står de länge i en kö, men hon försöker slappna av i den bekväma bilen. - When they go to the party they stand in a line for a long time, but she tries to relax in the comfortable car.

gå = to go on foot, or more abstractly “to go/attend” (e.g. gå till skolan, gå på fest).
åka = to go/travel using some kind of vehicle (car, bus, train, etc.).

Because they are in a bil (car), Swedish uses åker: they “go by vehicle” to the party.

Why is åker in the present tense when this can be about a future event?

Swedish very often uses the present tense to talk about the future, especially with time expressions or clauses like när (“when”).
När de åker till festen … can mean “When they go / when they are going to the party (later) …”.
You could say När de ska åka till festen …, but the simple present åker is completely normal for a (near) future here.

Why is the word order … festen står de länge i en kö … and not … festen de står länge …?

Swedish main clauses follow the “verb-second” (V2) rule: the finite verb must be in second position.
Here, När de åker till festen is a whole clause placed first; after that, the main-clause verb står must come next:

När de åker till festen (1st position) står (2nd) de (3rd) länge i en kö …

If you start with the subject instead, you get: De står länge i en kö när de åker till festen. The verb står is still in second position.

Why is it står de länge i en kö and not something with är (“are”) in the queue?

In Swedish you normally use posture verbs like stå (stand), sitta (sit), ligga (lie) instead of vara (är) to describe how someone is positioned.
The natural phrase is stå i kö = “to stand in line / in a queue”.
So de står länge i en kö literally means “they stand for a long time in a queue”, which is the standard idiom.

What does länge mean exactly, and how is it different from lång?

lång is an adjective: “long” (in length or duration), e.g. en lång film (“a long movie”).
länge is the adverb: “for a long time”, e.g. De väntar länge (“They wait for a long time”).
So de står länge i en kö = “they stand in a line for a long time”.

Why is it i en kö and not some other preposition, and why en?

The idiomatic expression is stå i kö (“stand in line”), so the preposition is i.
Kö is a common gender (“en‑word”) noun, so the indefinite form is en kö (“a queue/line”).
The definite form would be kön (“the queue”), as in De står längst bak i kön (“They are standing at the very back of the line”).

Why is it till festen and not just till fest or till en fest?

Fest is an “en‑word”, with:

en fest = “a party” (any party),
festen = “the party” (a specific one already known in the context).

Till festen means “to the (particular) party”.
Till en fest would be “to a party” (not specified which), and till fest is more of a fixed expression meaning “for partying / in a party context” (e.g. klädd till fest = “dressed for a party”).

Why do we say i den bekväma bilen and not i bekväm bil or i bekväm bilen?

When a noun is definite and has an adjective, Swedish uses “double definiteness”:

article + adjective (with definite form) + noun (with definite ending).

So you get:

indefinite: en bekväm bil – “a comfortable car”
definite: den bekväma bilen – “the comfortable car”

Bekväm bilen is incorrect, and bekväm bil would be indefinite (“comfortable car” in general).

Why does the adjective become bekväma and not stay as bekväm?

Adjectives in Swedish agree with the noun in definiteness and number.
For definite forms (and for all plurals), the adjective takes an -a ending:

en bekväm bil – “a comfortable car” (indefinite, singular)
den bekväma bilen – “the comfortable car” (definite, singular)
bekväma bilar – “comfortable cars” (plural)

So in den bekväma bilen, bekväma is the required definite form of the adjective.

Why is there no att before slappna av in hon försöker slappna av?

The basic infinitive is att slappna av (“to relax”).
With many verbs that express intention or start/stop (like börja, sluta, försöka), att is often optional: both Hon försöker att slappna av and Hon försöker slappna av are grammatically correct.
In modern spoken and everyday written Swedish, att is usually dropped here, so hon försöker slappna av is the most natural version.

What kind of verb is slappna av, and can I split it?

Slappna av is a particle verb (verb + small word/particle).
Here, av is not a normal preposition; it belongs tightly to slappna and together they mean “to relax”.
You normally keep them together: Hon slappnar av, Hon vill slappna av lite.
You cannot say avslappna in this meaning, and slappna on its own doesn’t mean “relax”.

What is the difference between de and hon here, and how are de/dem pronounced?

De is the subject pronoun “they”; hon is “she”. In this sentence, de refers to a group of people, and hon highlights one particular woman in or associated with that group.
In standard writing you use de (subject) and dem (object), but both are usually pronounced dom in everyday speech.
So you would say something like: När dom åker till festen står dom länge i en kö, men hon försöker slappna av i den bekväma bilen.