Learn Hardcore Icelandic: Hún lætur hundinn sofa inni þegar hún er einmana. - She lets the dog sleep inside when she is lonely.

Lætur is the 3rd person singular present form of the verb láta. In this sentence it is used in a causative construction, which is very common in Icelandic:

láta + (accusative object) + infinitive verb

Rough meaning: to let / to have / to make (someone) do something.

So:

Hún lætur hundinn sofa inni
Literally: She lets / makes the dog sleep inside.

The important points:

Hundinn is the thing being caused to do the action.
Sofa is an infinitive (the action that the dog is doing).
Læta here is not about physically putting the dog somewhere; it is about allowing or causing the dog to sleep inside.

Why is it hundinn and not hundur?

The base word is hundur (dog, nominative singular). In this sentence, hundinn is used because:

It is the direct object of lætur (and of sofa in the causative construction), so it must be in the accusative case.
It is definite: the dog, not a dog.

Declension of hundur (singular, masculine):

Nominative (subject): hundur – a dog
Accusative (object): hund – a dog (object form)
Nominative definite: hundurinn – the dog (as subject)
Accusative definite: hundinn – the dog (as object)

So in this sentence:

We need accusative (object)
We need definite (the dog)

→ hundinn is the correct form.

Why is sofa in the infinitive and not a conjugated form like sefur?

Because of the láta + object + infinitive pattern.

After læta in this causative use, the next verb stays in the infinitive:

Hún lætur hundinn sofa inni.
She lets/makes the dog sleep inside.

You would not say:

✗ Hún lætur hundinn sefur inni.

That would be ungrammatical, because you would then have two finite (conjugated) verbs (lætur and sefur) in the same simple clause, which this structure does not allow.

Think of læta as carrying the tense/person, and sofa just naming the action the dog is made to do.

What is the difference between inn and inni, and why is inni used here?

Both relate to being or going inside, but they express different directions:

inn = to the inside (movement, direction inwards)
inni = inside (location, being in)

Some examples:

Hún fer inn. – She goes in / inside. (movement)
Hún er inni. – She is inside. (location)

In the sentence:

Hún lætur hundinn sofa inni …
The dog is sleeping in a place (inside), not moving in, so the locational form inni is used.

If you wanted to emphasize movement, you might say something like:

Hún lætur hundinn fara inn. – She makes/lets the dog go in.

Why do we repeat hún in þegar hún er einmana? Could we leave it out?

In Icelandic, you generally cannot drop the subject pronoun the way you sometimes can in other languages. So you need hún in both clauses:

Hún lætur hundinn sofa inni – main clause
þegar hún er einmana – subordinate clause (when she is lonely)

Dropping hún in the second clause:

✗ … þegar er einmana

would be ungrammatical in normal Icelandic.

Also, repeating hún makes it clear that the person who is lonely is the woman, not the dog. The dog is hundurinn (masculine), so a pronoun referring to the dog would be hann, not hún.

Does einmana agree with hún in gender, number, or case?

Einmana is special: it is an indeclinable adjective (or adjective-like form). That means:

It does not change for gender, number, or case.
It looks the same with hún, hann, það, in singular and plural.

Examples:

Ég er einmana. – I am lonely.
Hann er einmana. – He is lonely.
Þau eru einmana. – They are lonely.

So in þegar hún er einmana, you do not change the form to match feminine or any case; einmana simply stays as it is.

What is the difference between einmana and ein / einn?

Both are common and easy to mix up.

ein / einn / eitt (depending on gender) usually means alone or the numeral one
einmana means lonely

So:

Hún er ein. – She is alone. (No one else is there with her.)
Hún er einmana. – She is lonely. (She feels lonely.)

In your sentence:

… þegar hún er einmana.
This tells us about her emotional state (lonely), not simply that she is physically alone.

You could say:

Hún lætur hundinn sofa inni þegar hún er ein.

but that would mean when she is alone (by herself) – which is related, but not exactly the same as when she is lonely.

Could we also say Hún leyfir hundinum að sofa inni? What is the difference from Hún lætur hundinn sofa inni?

Yes, you can say:

Hún leyfir hundinum að sofa inni.

Main differences:

Verb meaning and tone
- leyfa = to allow, to permit (neutral, explicit permission)
- láta = to let / make / have (broader: allowing, causing, arranging)
Hún leyfir hundinum að sofa inni focuses on permission: she allows him to do it.
Hún lætur hundinn sofa inni can feel a bit more neutral: she has him sleeping inside / lets him sleep inside (could be routine, arrangement, habit).
Grammar differences
With leyfa:
- The dog is dative, because leyfa takes a dative indirect object: hundinum
- You use að + infinitive: að sofa
Pattern: leyfa + (dative) + að + infinitive
→ Hún leyfir hundinum að sofa inni.
With láta:
- The dog is accusative: hundinn
- No að before the infinitive: sofa directly
Pattern: láta + (accusative) + infinitive
→ Hún lætur hundinn sofa inni.

Both are correct; they just use different verbs and slightly different grammatical constructions.

Why is it hundinn (accusative) with láta and hundinum (dative) with leyfa?

This is about which case each verb governs:

láta in the causative pattern takes a direct object in the accusative:
- Hún lætur hundinn sofa inni.
  - hundinn = accusative definite (the dog)
  - He is the one being made/let to do the action.
leyfa usually takes a dative indirect object, plus an að + infinitive clause:
- Hún leyfir hundinum að sofa inni.
  - hundinum = dative definite
  - He is the beneficiary of the permission.

So the choice of verb (láta vs leyfa) automatically changes what case the noun should be in.

How does word order work around sofa inni? Could you say Hún lætur hundinn inni sofa?

The natural, neutral word order here is:

Hún lætur hundinn sofa inni.

That is: subject – verb – object – main verb (infinitive) – adverb (inni).

You could move inni a bit, but Hún lætur hundinn inni sofa sounds odd or at least very marked; Icelandic usually keeps adverbs like inni after the infinitive in such constructions.

More natural variants:

Hún lætur hundinn alltaf sofa inni. – She always lets the dog sleep inside.
Hún lætur hundinn sofa inni á nóttunni. – She lets the dog sleep inside at night.

Adverbs that modify how/where the action of sofa happens typically come after sofa.

How is this sentence pronounced, especially æ, nn, and einmana?

Very roughly in IPA:

Hún lætur hundinn sofa inni þegar hún er einmana.
/huːn ˈlaiːtʏr ˈhʏntɪn ˈsɔːva ˈɪnːɪ ˈθjɛːɣar huːn ɛr ˈeimˌmana/

Key points:

ú in Hún: long u sound /uː/.
æ in lætur: like English eye, /ai/.
nn in inni: often a long /nː/.
ð is not present here; þegar starts with þ, pronounced like English th in thing.
einmana: stress on the first syllable ein-: EIN-mana, with ein pronounced like ane or ain (/ei/ sound).

Stress in Icelandic almost always falls on the first syllable of each word: Hún, LÆ-tur, HUN-dinn, SO-fa, IN-ni, ÞE-gar, Hún, er, EIN-mana.