Learn Hardcore Icelandic: Hún skrifar í dagbókina sína á kvöldin til að muna fyrir hvað hún er þakklát. - She writes in her diary in the evenings to remember what she is grateful for.

In the sentence, dagbókina is accusative definite singular, because it is the direct object of skrifar and appears after the preposition í, which here takes the accusative (see below).

Why is the possessive after the noun: dagbókina sína instead of sína dagbók?

In Icelandic, possessive pronouns normally come after the noun they modify:

bíllinn minn = my car
húsið hennar = her house
dagbókina sína = her (own) diary

Putting the possessive before the noun (sína dagbók) is possible but much more marked, and would sound poetic or emphatic. The neutral, everyday word order is noun + possessive.

What is the difference between sína and hennar for her?

Both can translate as her, but they are used differently:

sinn / sín / sitt (here sína) is the reflexive possessive. It refers back to the subject of the same clause.
hennar is the non‑reflexive possessive, used when the possessor is someone else.

In this sentence, the subject is hún (she), and the diary belongs to that same hún, so Icelandic uses the reflexive:

Hún skrifar í dagbókina sína…
= She writes in her own diary (the subject’s diary).

If we said:

Hún skrifar í dagbókina hennar…

it would usually mean: She writes in *her diary (some other woman’s diary).*

Why is it sína specifically? What form of the pronoun is that?

The reflexive possessive pronoun declines to match the gender, number, and case of the noun it modifies. Here:

Noun: dagbókina
- gender: feminine
- number: singular
- case: accusative

So the matching form of sín is sína (feminine singular accusative):

sín – base form (fem. nom.)
sína – fem. acc. sg. (used with dagbókina)

Why is it í dagbókina and not í dagbókinni?

The preposition í can take either accusative or dative, with a difference in meaning:

í + accusative – usually into, or putting/placing something in
í + dative – usually in, a static location

Here, skrifa í dagbókina is understood as writing into the diary, so í takes the accusative:

í dagbókina (acc.) = into the diary
í dagbókinni (dat.) = in the diary (more static sense; can occur, but the accusative is very common with skrifa í)

So the form dagbókina matches the accusative required by í in this usage.

What does á kvöldin literally mean, and why does it translate as in the evenings?

Literally:

á = on / at / in
kvöld-in = the evenings (neuter plural definite; nom./acc. form is the same)

The expression á kvöldin is an idiomatic time phrase meaning in the evenings / at night (in the evenings). Icelandic often uses the definite with recurring times:

á morgnana = in the mornings
á kvöldin = in the evenings
á veturna = in (the) winters

So á kvöldin describes a habitual time: she does this regularly in the evenings.

Why is the verb skrifar in the present tense, not something like “is writing”?

Icelandic uses the simple present to express both:

general truths
regular, repeated habits

English also uses the simple present for habits: She writes in her diary in the evenings. There is no separate progressive form (is writing) in this sentence because we are not describing an action happening right now, but a regular activity.

So Hún skrifar… = She writes… (habitually).

What does the phrase til að muna mean, and why is til að used?

muna = to remember
að here is the infinitive marker (“to remember”)
til is a preposition that, together with að + infinitive, forms a purpose construction.

til að + infinitive = in order to / to (for the purpose of)

So:

til að muna = in order to remember / to remember (as a purpose)

The whole segment …á kvöldin til að muna… means in the evenings in order to remember…

Why is the word order fyrir hvað hún er þakklát and not like English hvað hún er þakklát fyrir?

In Icelandic, prepositions almost never “strand” at the end of a clause. They must come before their object, including interrogative words like hvað.

English allows: what she is grateful for (preposition at end)
Icelandic must say: fyrir hvað hún er þakklát = for what she is grateful

So:

fyrir (for) + hvað (what) stay together: fyrir hvað
Then you get the embedded clause: hún er þakklát (she is grateful)

*hvað hún er þakklát fyrir is ungrammatical or very marked in Icelandic.

Why is the word order in the embedded clause hún er þakklát, not er hún þakklát?

In main clauses, Icelandic is generally verb‑second (V2):

Hún er þakklát. = She is grateful.
(Subject hún, then verb er.)

In embedded clauses introduced by words like hvað, að, hvort, etc., the usual order is subject before verb:

fyrir hvað hún er þakklát
(= for what she is grateful)

Putting the verb before the subject here (fyrir hvað er hún þakklát) would sound like a direct question, not an embedded one, and would not fit in this sentence.

What does þakklát mean exactly, and why does it have this form (and not þakklátur)?

þakklátur is the masculine form of the adjective grateful.
þakklát is the feminine form (nominative singular).

Adjectives in Icelandic agree with the noun (or pronoun) they describe in gender, number, and case. The subject here is hún (she), which is feminine singular nominative, so the adjective is:

hún er þakklát = she is grateful
hann er þakklátur = he is grateful
barnið er þakklátt = the child is grateful (neuter)

So þakklát matches the feminine pronoun hún.

Could we change the word order of the main sentence, for example: Hún skrifar á kvöldin í dagbókina sína…?

Yes. Icelandic word order is relatively flexible for adverbials (time, place, manner) as long as you keep the verb in second position in main clauses and the internal phrases intact. These are all acceptable:

Hún skrifar í dagbókina sína á kvöldin…
Hún skrifar á kvöldin í dagbókina sína…

Both mean the same thing. Slight differences in emphasis are possible (e.g. putting á kvöldin earlier can emphasize when she writes), but grammatically both are fine.

Is there any difference in meaning between á kvöldin til að muna… and English in the evenings so that she can remember…?

Functionally, they are very close:

á kvöldin = in the evenings (habitual time)
til að muna = in order to remember / so that she remembers

English often uses so that or in order to; Icelandic commonly uses til að + infinitive. Both structures express purpose: she writes for the purpose of remembering what she is grateful for.