Afmælisbarnið blæs á kertin og hlær hátt.

Afmælisbarnið means the birthday child (i.e., the person whose birthday it is).

It’s a compound:

• afmæli = birthday
• barn = child
• plus the definite ending -ið because barn is neuter: barnið = the child

So:

• afmælisbarn = birthday child (indefinite)
• afmælisbarnið = the birthday child (definite)

Icelandic often uses the definite form when you mean a specific, known person/thing—similar to English the.

Here, in a birthday context, there is typically one salient person: the birthday child/person, so Afmælisbarnið is natural.

Indefinite would be possible in a different context:

• Afmælisbarn blæs á kertin. = A birthday child blows on the candles (more generic/odd in a normal birthday scene)

blæs is present tense, 3rd person singular.

Dictionary form (infinitive): blása = to blow

A few useful forms:

• infinitive: blása
• present (he/she/it): blæs
• past: blés
• past participle: blásið

Because Icelandic commonly expresses blow on (something) as blása á (eitthvað).

So:

• blása á kertin = blow on the candles

Without á, blása is more like to blow (air) in a general sense, and you’d typically still need some structure to show what’s being affected.

Related expressions you may see:

• blása út = blow out (e.g., a candle/light)
• blása í = blow into (something)

kerti means candle, and it’s a neuter noun.

• kerti = a candle / candles (context-dependent, because the plural is the same form)
• kertin = the candles (definite plural)

Here it’s a specific set of candles (typically the ones on the cake), so the definite form kertin is used.

After á, the case depends on meaning:

• á + accusative often expresses direction/target (movement toward / action onto)
• á + dative often expresses location (being on)

In blása á, the candles are the target of the blowing, so á takes the accusative.

For kerti, nominative and accusative plural look the same (kerti), and the definite ending -n gives kertin, which works here as accusative plural definite.

Because the subject Afmælisbarnið applies to both coordinated verbs:

• Afmælisbarnið blæs á kertin og hlær hátt. = The birthday child blows on the candles and laughs loudly.

This is like English: you don’t normally repeat the subject unless you want emphasis or contrast.

hlær is the present tense (3rd person singular) of hlæja = to laugh.

It is somewhat irregular in its past tense:

• infinitive: hlæja
• present: hlær
• past: hló
• past participle: hlegið

hátt is the adverbial use of the adjective hár (high, and by extension loud in certain expressions).

Icelandic often uses the neuter singular form as an adverb-like form, especially in fixed patterns:

• hlæja hátt = laugh loudly
• tala hátt = speak loudly

So hátt here functions like an adverb even though it looks like an adjective form.

In main clauses Icelandic typically follows a V2 (verb-second) pattern: the finite verb tends to be in the second “slot.”

Here: 1) Afmælisbarnið (subject) 2) blæs (finite verb) then the rest: á kertin og hlær hátt

So yes, this sentence fits the common main-clause pattern.

A practical pronunciation guide (approximate):

• Afmælisbarnið: stress is on the first syllable: AF-mæ-lis-bar-nith
  
  • æ is like i in bite (often close to ai)
  • final -ið is often like a soft -ith sound (the ð is very light)
• blæs: roughly blaiss (with æ ~ ai)
• hlær: the hl- starts with a voiceless l (a “breathy l”); roughly lair (again æ ~ ai)