Learn Hardcore Icelandic: Þessi dagur er sérstakur fyrir afa minn.

Þessi – this (demonstrative pronoun/adjective, masculine singular nominative)
dagur – day (noun, masculine singular nominative)
er – is (3rd person singular of að vera – to be)
sérstakur – special, particular, unusual (adjective, masculine singular nominative)
fyrir – for (preposition, here meaning for / to the benefit of / in relation to)
afa – grandfather (noun afi, here in the accusative singular)
minn – my (possessive pronoun, masculine singular accusative, agreeing with afa)

So structurally: This day is special for my grandfather.

Why is it Þessi dagur and not Þetta dagur?

In Icelandic, demonstratives must agree in gender, number, and case with the noun they modify.

dagur (day) is masculine.
The masculine nominative singular form of þessi is Þessi.
Þetta is the neuter nominative/accusative singular form.

So:

Þessi dagur = this day (correct: masculine–masculine agreement)
Þetta dagur = ❌ incorrect, because þetta is neuter but dagur is masculine.

If the noun were neuter, like barn (child), you would say Þetta barn (this child), but with dagur you must use Þessi.

Why is it Þessi dagur, not Þessi dagurinn? Can you also say Þessi dagurinn?

Both are grammatically possible, but there is a nuance:

Þessi dagur – this day, introducing or talking about some day in a more neutral way.
Þessi dagurinn – roughly this (particular) day, with an added sense that the day is already known, specific, or strongly focused.

In Icelandic, the definite article is usually attached to the noun:

dagur – a day / day
dagurinn – the day

With a demonstrative:

Þessi dagur – common, quite neutral: this day.
Þessi dagurinn – more emphatic, like this particular/the very day.

In your sentence, Þessi dagur er sérstakur fyrir afa minn, the neutral Þessi dagur is natural and idiomatic. Þessi dagurinn would sound more pointed, as if contrasting this specific day with others already in the conversation.

Why does dagur end in -ur instead of just dag?

Dagur is the nominative singular form of the masculine noun dagur (day). Icelandic nouns change form according to case:

A common paradigm for dagur:

Nominative (subject): dagur – Þessi dagur er sérstakur.
Accusative (direct object): dag – Ég elska þennan dag. (I love this day.)
Dative: degi – Ég hugsa um þennan dag. (I think about this day.)
Genitive: dags – á þessa dags morgni (on this day’s morning)

In your sentence, day is the subject of the verb er (is), so it must be in the nominative, hence dagur with -ur.

Why does sérstakur have the ending -ur here?

Adjectives in Icelandic must agree with the noun they describe in gender, number, and case.

The noun is dagur – masculine, singular, nominative.
The adjective sérstakur (special) is used as a predicate adjective (after er).
Predicate adjectives also take the nominative and agree with the subject.

So:

Masculine nominative singular of sérstakur is sérstakur (ending -ur).
If the noun were feminine or neuter, the form would change, for example:
- Feminine: Þessi vika er sérstök. – This week is special.
- Neuter: Þetta kvöld er sérstakt. – This evening is special.

Here, Þessi dagur → masculine nominative → adjective: sérstakur.

What is the difference between sérstakur and sérstaklega?

sérstakur is an adjective:
- Þessi dagur er sérstakur.
  This day is special.
sérstaklega is usually an adverb, meaning especially / particularly:
- Þessi dagur er sérstaklega mikilvægur.
  This day is especially important.

You use:

sérstakur / sérstök / sérstakt to describe a thing as special.
sérstaklega to modify an adjective/verb: especially / particularly.

So your sentence correctly uses the adjective sérstakur to describe dagur.

Why is it fyrir afa minn and not fyrir afi minn?

The preposition fyrir usually governs the accusative case when it has the meaning for / for the benefit of / concerning.

The noun afi (grandfather) has:
- Nominative: afi
- Accusative: afa

Because fyrir is followed by an object in the accusative, we must use:

fyrir afa – for (my) grandfather
not nominative fyrir afi.

So fyrir afa minn is for my grandfather, with both afa and minn in the accusative.

What are the main forms of the noun afi (grandfather)?

For afi (a common masculine noun), the singular forms are:

Nominative: afi – subject
- Afi er heima. – Grandfather is at home.
Accusative: afa – object (here after fyrir)
- Ég hringi í afa. – I call grandfather.
Dative: afa – many prepositions
- Ég tala við afa. – I talk to grandfather. (Note: við takes accusative, but many others take dative.)
Genitive: afa – possession
- Húsið afa – grandfather’s house.

So afa appears in several oblique cases; here it is accusative because of fyrir.

Why is minn after afa (afa minn) instead of before, like minn afi?

In Icelandic, the neutral and most common position for a possessive pronoun is after the noun:

afi minn – my grandfather
bókin mín – my book
húsið mitt – my house

So:

fyrir afa minn – for my grandfather is the usual, natural order.

You can put the possessive before the noun (minn afi), but that tends to sound more emphatic, stylistic, or poetic, rather than everyday neutral speech.

In this sentence, afa minn is the standard, idiomatic choice.

Why is it minn and not mínum or míns?

The form of the possessive pronoun minn must agree with the noun it modifies in gender, number, and case.

afa is masculine singular accusative (because of fyrir).
So minn must also be masculine singular accusative.

The masculine singular forms of minn are typically:

Nominative: minn
Accusative: minn
Dative: mínum
Genitive: míns

Since we need accusative, the correct form is minn, not mínum (dative) or míns (genitive).

So:

fyrir afa minn – for my grandfather (accusative)
If it were dative, e.g. after a dative preposition, you might see:
- með afa mínum – with my grandfather (dative).

Could I say fyrir minn afa instead of fyrir afa minn?

Grammatically, fyrir minn afa is possible, and minn is still in the accusative, agreeing with afa.

However:

fyrir afa minn is more natural and neutral.
fyrir minn afa sounds more emphatic or marked, often with a subtle contrast (e.g., for my grandfather, not someone else’s), or more literary.

For everyday speech and standard style, fyrir afa minn is the preferred word order.

How do you pronounce Þ in Þessi and the main words in this sentence?

Rough pronunciation (in IPA) and English approximations:

Þessi – [ˈθɛsːɪ]
- Þ = [θ], like th in thing.
- Stress on the first syllable: ÞES-si.
dagur – [ˈtaːɡʏr]
- d often sounds closer to a soft t here, g is a hard [ɡ], u in ur is like a short u in pull, but fronted.
er – [ɛr]
- Similar to air but shorter.
sérstakur – [ˈsjɛrstaˌkʏr]
- sé = [sjɛ], like s
  - ye in yes.
- Stress on the first syllable: SJER-sta-kur.
fyrir – [ˈfɪːrɪr] or [ˈfɪrɪr]
- First vowel like i in bit, then a rolled or tapped r.
afa – [ˈaːva]
- f between vowels is voiced, so it sounds like v: ava.
minn – [mɪnː]
- Like min in minnow, but with a long n.

So the whole sentence roughly:
[ˈθɛsːɪ ˈtaːɡʏr ɛr ˈsjɛrstaˌkʏr ˈfɪːrɪr ˈaːva mɪnː]

Can the word order change, for example to Þessi dagur er fyrir afa minn sérstakur?

Icelandic has flexible word order, but there are preferred patterns.

Þessi dagur er sérstakur fyrir afa minn.
This is the most natural order:
Subject – Verb – Predicate adjective – Prepositional phrase.

Other orders:

Þessi dagur er fyrir afa minn sérstaklega mikilvægur.
Here the prepositional phrase comes before the adjective phrase; this is possible but sounds more marked or emphatic.

Your suggested Þessi dagur er fyrir afa minn sérstakur is understandable, but it’s less natural than placing sérstakur right after er in this short sentence. In simple statements, Icelandic typically keeps:

Subject – Verb – Complement (adjective) – Extra information (PPs, adverbs).