Learn Hardcore Icelandic: Þau finna ekki lykilinn sinn og þurfa að bíða í bílnum. - They cannot find their key and need to wait in the car.

þeir = they (masculine-only group)
þær = they (feminine-only group)
þau = they (neuter plural), used for mixed-gender or unspecified groups, and also for things.
Here, þau is the default when the group isn’t all-male or all-female (or when you just don’t want to specify).

Can þau be used like English singular “they” for one person?

Traditionally, no: þau is plural. In contemporary usage, some people do use þau for a nonbinary person, but that’s not yet standard in formal Icelandic. You’ll also see the gender-neutral singular hán in some contexts.

Why does ekki (not) come after the verb finna?

Main clauses follow a “verb-second” pattern: the finite verb is in second position, and ekki typically comes right after it.

Correct: Þau finna ekki …
Not correct: Þau ekki finna …
After the conjunction og, the second clause again uses verb-second: … og þurfa að bíða …

Why is it lykilinn, not something like lykillinn or lykil?

The base noun is lykill (key), masculine.
finna takes an object in the accusative case.
Accusative singular of lykill is lykil.
With the definite article, accusative singular becomes lykilinn (“the key”).
Note the spelling difference: nominative definite would be lykillinn, but accusative definite is lykilinn.

What does sinn add in lykilinn sinn?

sinn is the reflexive possessive “own,” referring back to the subject. lykilinn sinn means “their own key.”

lykilinn þeirra would mean “their key,” but possibly someone else’s key (not necessarily the subject’s).
Without any possessive (lykilinn), it’s just “the key” (context decides whose).

Why does sinn go after the noun, and does it have to agree with the noun?

Short possessives (minn, þinn, sinn) usually follow the noun, which also takes the definite ending: lykilinn sinn.
sinn must agree with the noun it modifies in gender, number, and case (not with the possessor). Here the noun is masculine accusative singular, so sinn is also masculine accusative singular: sinn.
Other forms you might see:

masculine dative singular: sínum (e.g., í bílnum sínum)
feminine accusative singular: sína
neuter accusative singular: sitt

Could I just say Þau finna ekki lykilinn and leave out sinn?

Yes, but then it’s simply “the key,” not clearly “their own key.” Using sinn removes that ambiguity.

Why is it þurfa að bíða and not just þurfa bíða?

Many Icelandic verbs that govern another verb take að before the infinitive. Þurfa (to need/have to) is one of them: þurfa að + infinitive (bíða).

Shouldn’t bíða usually take eftir? When do I use bíða eftir?

bíða eftir e-u = wait for something/someone.
bíða [staður] = wait [in/at a place].
In this sentence, they’re waiting in a location, so að bíða í bílnum is correct. If they were waiting for the key, you’d say bíða eftir lyklinum.

Why is it í bílnum (dative) and not í bílinn (accusative)?

The preposition í takes:

dative for location (where?): í bílnum = in the car
accusative for motion into (where to?): í bílinn = into the car
Here it’s a static location, so dative is required.

What form is bílnum, and how is it built?

The base noun is bíll (car), masculine.
Dative singular is bíl + definite ending → bílnum (“in the car”).
Other useful forms: nominative definite bíllinn, accusative definite bílinn.

Is finna ekki the normal way to say “can’t find”?

Yes. In context, present finna ekki often means “can’t find” (not finding successfully). For explicit inability, you can use geta ekki fundið (cannot manage to find), e.g., Þau geta ekki fundið lykilinn sinn.

Could the object come before ekki, like in Þau finna lykilinn sinn ekki?

That word order is unnatural. Neutral order places ekki right after the finite verb. You can move elements for emphasis in Icelandic, but you would not typically put the object right before ekki here. Stick with Þau finna ekki lykilinn sinn.

Why no possessive with “car”? Shouldn’t it be “in their car”?

Icelandic often relies on context and the definite article. í bílnum can naturally imply “in their car.” If you want to make it explicit, you can say í bílnum sínum (“in their own car,” with reflexive dative masculine singular sínum).

Any pronunciation tips for tricky letters here?

Þ in Þau = voiceless “th” [θ].
au in Þau is a diphthong roughly like [œy].
ð in bíða is the voiced “th” [ð] (never at the start of a word).
y in lykilinn is pronounced like short i [ɪ] (same as i in this word).
Double consonants (e.g., nn in finna) indicate a short preceding vowel.
kk in ekki is pronounced [hc] (a hard aspirated sound).

How would this look in the past tense?

Þau fundu ekki lykilinn sinn … = They didn’t/couldn’t find their key …
… og þurftu að bíða í bílnum. = … and had to wait in the car.
Key forms: past plural of finna → fundu; past plural of þurfa → þurftu.