Learn Hardcore Icelandic: Stelpuna langar ekki að sitja lengi í biðstofunni. - The girl does not want to sit long in the waiting room.

The verb langa is impersonal: the person who feels the desire is put in the accusative case, and the verb stays in 3rd person singular. There is no ordinary nominative subject here.

Example with a pronoun: Hana langar ekki að sitja… (Her = accusative).
With pronouns this pattern is: mig, þig, hann/hana, okkur, ykkur, þá/þær/þau
- langar (e.g., Mig langar…, Okkur langar…).

Can I say Stelpan langar… or Hún langar…?

Not in standard Icelandic. You should use the accusative: Stelpuna/Hana langar…. You may hear Mér langar… (dative) in colloquial speech, but it’s nonstandard; the prescribed form is Mig langar… (accusative).

Why is the verb in second position in Stelpuna langar…? Could I start the sentence with something else?

Icelandic main clauses are verb-second (V2). Whatever you put first (topic), the finite verb comes next.

You can say: Hana langar ekki…, Í biðstofunni langar stelpuna ekki…, or use an expletive: Það langar stelpuna ekki…. All keep the finite verb in second position.

What exactly is negated here, and where should ekki go?

In Stelpuna langar ekki að sitja…, ekki negates the wanting: she doesn’t want to sit. If you move ekki inside the infinitive, you can target the action or its manner:

Stelpuna langar að sitja ekki lengi… ≈ she wants to sit, but not for long (focus on a short duration).
With other verbs the contrast is clearer: Hana langar ekki að fara (she doesn’t want to go) vs. Hana langar að fara ekki (she wants not to go). The latter is possible but marked and less common.

Why lengi and not lengur?

lengi = for a long time (neutral duration). Example: Hún vill ekki bíða lengi (she doesn’t want to wait for a long time).
lengur = any longer/anymore or comparative “longer.” Example: Hún vill ekki bíða lengur (she doesn’t want to wait any longer). Your sentence means “not for a long time,” so lengi is right.

Why is it í biðstofunni (dative) and not í biðstofuna?

The preposition í is a two-way preposition:

Location (in/at) → dative: í biðstofunni (in the waiting room).
Direction (into) → accusative: í biðstofuna (into the waiting room).

What are the endings -una in stelpuna and -unni in biðstofunni?

Icelandic adds the definite article as a suffix:

stelpa (a girl) → accusative singular: stelpu; add definite article → stelpuna (the girl, acc.sg.).
biðstofa (a waiting room) → dative singular: biðstofu; add definite article → biðstofunni (the waiting room, dat.sg.).

What does að do before sitja? Can I use til að?

að marks the infinitive (“to sit”). With langa, both að and til að are used before a verb:

Hana langar að sitja… (very common)
Hana langar til að sitja… (also fine; sometimes a touch more formal or explicit) Both are acceptable here.

Is að sitja the right verb, or should it be að setjast?

sitja = to sit (be in a sitting position). Use this for duration: að sitja lengi.
setjast (niður) = to sit down (the action of moving into a sitting position). Use this for the moment of sitting down: Hún settist (niður) í biðstofunni.

Could I rephrase this with vilja (to want)? What’s the difference between langar and vill?

Yes: Stelpan vill ekki sitja lengi í biðstofunni.

vilja takes a nominative subject (Stelpan vill…) and often sounds stronger/volitional (“intend, want”).
langa expresses desire/feeling (“feel like, would like”) and puts the experiencer in the accusative (Stelpuna langar…). In polite requests, Mig langar… can map to “I’d like…”, while Ég vil… can sound more forceful.

Why is í used here and not á? Do rooms usually take í?

For being inside an enclosed space like a room, í is standard: í biðstofunni (in the waiting room). á is used for surfaces and certain named places (e.g., á skrifstofunni “at the office” is also common), but with biðstofa the natural choice is í.

Are there other Icelandic verbs that put the experiencer in a non‑nominative case like langa?

Yes, several:

líka (to like): experiencer in dative. Mér líkar þetta.
vanta (to need/lack): experiencer in accusative. Mig vantar peninga.
kalt/heitt with vera (to be cold/hot): dative experiencer. Mér er kalt.
dreyma (to dream): accusative experiencer. Mig dreymdi undarlegan draum. These are often called “quirky subject” or impersonal patterns.