Learn Hardcore Icelandic: Hann lokar öðru auganu þegar sólin skín beint í það. - He closes one eye when the sun shines directly into it.

So „Hann lokar öðru auganu“ literally is “He closes the other eye”, but it naturally means “He closes his other eye”.
You only add „hans“ (his) if you need to avoid ambiguity, e.g. if it could be someone else’s eye.

Why is it „öðru“ and not „annað“ or „eitt“ before „auganu“?

„annar“ means “other, second, the other (of two)”, not just “another” in the loose English sense.
Its neuter forms are:
- Nominative: annað
- Accusative: annað
- Dative: öðru
- Genitive: annars

Here we have dative (because of the verb „loka“, see another question), so we must use „öðru“, the dative neuter singular form.

So the phrase „öðru auganu“ is literally “the other eye (dative)”, i.e. “(in) the other eye / (to) the other eye”.

You could in principle use „eitt augað“ (“one of the eyes”), but that would sound different in nuance and is not what the original sentence uses.

What case is „öðru auganu“, and why that case?

„Öðru auganu“ is in the dative singular neuter.

„auga“ = eye (neuter noun)
Dative singular, definite: „auganu“
The adjective/pronoun „annar“ must match that: „öðru“ (dative neuter singular).

The reason is the verb:

loka + dative
„Hann lokar hurðinni.“ – He closes the door. (hurðinni = dative)
„Hann lokar glugganum.“ – He closes the window. (glugganum = dative)

So with „auga“:

„Hann lokar öðru auganu.“ – He closes the other eye.

If you used nominative (annað augað), it would be grammatically wrong with loka.

What exactly is going on with „auganu“? How does „auga“ decline?

„Auga“ is a neuter noun with a somewhat regular pattern. Singular:

Nominative: auga – (an) eye
Accusative: auga
Dative: auga
Genitive: auga

With the definite article:

Nominative/Accusative: augað – the eye
Dative: auganu
Genitive: augans

So „auganu“ = “to/in the eye” (dative, definite).

The -nu in „auganu“ is the dative definite ending for many neuter nouns.

What does „það“ refer to in „beint í það“ – the sun or the eye?

It refers to the eye, not the sun.

„augað“ (the eye) is neuter → pronoun: það
„sólin“ (the sun) is feminine → pronoun in accusative would be hana

So „í það“ can only naturally refer to a neuter word like „augað“, not to „sólin“.

Meaning: the sun shines directly into the eye (i.e. his eye).

Why is it „í það“ and not „í því“?

The preposition „í“ can take accusative or dative:

Accusative: direction / into / onto
Dative: location / in / inside

Here the idea is “shines directly into it” → a sense of direction, so accusative is used:

Neuter pronoun:
- Nominative: það
- Accusative: það
- Dative: því

Hence „í það“ (accusative), not „í því“.

Why is it „sólin“ with the definite ending, instead of just „sól“?

In Icelandic, unique natural objects like the sun, the moon, the Earth are normally referred to with the definite form:

„sólin“ – the sun
„mánuðurinn“ / „tunglið“ – the moon
„jörðin“ – the Earth

So „sólin skín“ is the natural way to say “the sun shines”. Using bare „sól“ here would sound odd or poetic, not the normal everyday wording.

What kind of word is „beint“, and what does it add?

„Beint“ is an adverb meaning “directly, straight”.

It comes from the adjective „beinn“ (“straight”), whose neuter form is „beint“; neuter singular often serves as an adverb in Icelandic.

In „sólin skín beint í það“, it specifies how the sun shines into the eye: directly, not at an angle or through reflection.

How does word order work in „Hann lokar öðru auganu þegar sólin skín beint í það.“?

Two clauses:

Main clause – verb-second (V2):
- „Hann lokar öðru auganu“
  - Subject: Hann
  - Verb: lokar (2nd position)
  - Rest: öðru auganu
Subordinate clause with „þegar“ (“when”):
- „þegar sólin skín beint í það“
  - Subordinating conjunction: þegar
  - Subject: sólin
  - Verb: skín
  - Adverbial: beint í það

Unlike German, Icelandic subordinate clauses usually keep a normal SVO order after the conjunction, so „þegar sólin skín …“ is the default, not „þegar sólin beint í það skín“ or verb-final.

Why is the present tense used in both clauses? Could this mean “when the sun is shining”?

Yes, the Icelandic simple present often covers both English:

“shines” (simple present)
“is shining” (present progressive)

Here, „Hann lokar öðru auganu þegar sólin skín beint í það.“ describes a habitual/general behavior:

Whenever the sun shines directly into his eye, he closes one eye.

Icelandic has no separate progressive tense (like “is shining”), so the present does the job.

Is „loka“ always used with the dative? Can you give more examples?

Yes, „loka“ governs the dative case for its object.

Examples:

„Geturðu lokað glugganum?“ – Can you close the window? (gluggi → glugganum, dat.)
„Hún lokaði bókinni.“ – She closed the book. (bók → bókinni, dat.)
„Við lokum dyrunum klukkan tíu.“ – We close the doors at ten. (dyr → dyrunum, dat. pl.)

So „Hann lokar öðru auganu“ fits this pattern: the thing being closed is in the dative.

Could you also say „annað augað“ instead of „öðru auganu“ here?

Not with „loka“ in this exact sentence, because of the case:

„annað augað“ = nominative/accusative (the other eye)
„öðru auganu“ = dative (required by loka)

You could see „annað augað“ in another grammatical role, for example:

„Annað augað er stærra en hitt.“ – The other eye is bigger than the other one. (subject → nominative)

But with „loka“, the object must be dative, so you need „öðru auganu“.

Can I move parts of the sentence around, like start with the „when“-clause?

Yes, you can front the „þegar“-clause, but the V2 rule will still apply to the main clause:

„Þegar sólin skín beint í það, lokar hann öðru auganu.“
- Subordinate clause first
- Then main clause, with verb in 2nd position: lokar hann …

Both word orders are natural:

„Hann lokar öðru auganu þegar sólin skín beint í það.“
„Þegar sólin skín beint í það, lokar hann öðru auganu.“

The meaning stays the same; the second version just emphasizes the condition (“when the sun shines…”) a bit more.