Leigjandinn á þriðju hæð veit ekki nákvæmlega hvenær viðgerðin byrjar.

-inn is the definite article suffix attached to the noun.

• leigjandi = a tenant (indefinite)
• leigjandinn = the tenant (definite)
  This is one of the main ways Icelandic marks “the” (instead of using a separate word like English).

It’s nominative singular (masculine), because it’s the subject of the main verb veit (knows).
A common clue: dictionary form leigjandi is nominative, and the definite nominative singular is leigjandinn.

Because á with a location meaning (“on/at” a place) typically takes the dative in Icelandic.

• hæð is feminine, and here it’s dative singular: hæð (same spelling as nominative, but the case is different)
• The ordinal þriðji (third) becomes þriðju because it must agree with hæð in gender (feminine), number (singular), and case (dative).

So: á þriðju hæð = on the third floor.

Both can occur, but they differ in style/feel:

• á þriðju hæð is very common and often feels like a fixed “address-like” phrase: on the third floor. The ordinal already makes it specific, so the noun is often left without the separate definiteness marking.
• á þriðju hæðinni is also correct and can feel a bit more explicit/pointing: on the third floor (the one we’re talking about).

In everyday usage, á þriðju hæð is extremely natural.

Icelandic is a V2 language in main clauses: the finite verb tends to be the second “slot” in the clause.

Here the first slot is the subject Leigjandinn á þriðju hæð (the subject plus its location phrase), and then the verb comes:

• Leigjandinn á þriðju hæð (slot 1)
• veit (slot 2)

So it still satisfies the V2 pattern.

In Icelandic, ekki (not) normally comes after the finite verb in a main clause:

• veit ekki = does not know

This is a very typical negation placement, especially with simple verb forms.

nákvæmlega is an adverb meaning exactly / precisely. It commonly appears:

• after ekki: veit ekki nákvæmlega …
  or sometimes in slightly different positions depending on emphasis, but the given placement is very neutral and natural.

Because hvenær introduces a subordinate clause here (“when …”), not a direct question.

Main clause:

• Leigjandinn … veit ekki nákvæmlega …

Subordinate clause:

• hvenær viðgerðin byrjar = when the repair begins

In Icelandic subordinate clauses, you usually get subject + verb order (no V2 inversion), so viðgerðin comes before byrjar.

viðgerðin is nominative singular definite (feminine):

• viðgerð = repair (a repair job)
• viðgerðin = the repair

It’s nominative because it’s the subject of byrjar in the subordinate clause.

byrjar is the present tense, 3rd person singular form of the verb byrja (to begin/start).

• infinitive: að byrja
• present (3sg): (hann/hún/það) byrjar

Icelandic splits “know” into different verbs:

• vita (veit) = know a fact / be aware of information
  
  • fits here: not knowing when something starts is not knowing information.
• kunna (kann) = know how / be able; know (a language)
  
  • e.g. Ég kann íslensku.
• þekkja (þekkir) = be acquainted with / know (a person/place)
  
  • e.g. Ég þekki hann.

So veit is the natural choice.

In modern Icelandic, a comma before a subordinate clause like hvenær viðgerðin byrjar is often not used (and many writers omit it). Older or more conservative comma habits might insert one, but the no-comma version is very common and acceptable in contemporary writing.

Key pronunciation points:

• þ = voiceless th like in think
  
  • þriðju
• ð = voiced th like in this (often very soft, and sometimes not strongly pronounced)
  
  • appears inside words like viðgerðin
• ei in veit sounds like a long vowel roughly like “ay” (but Icelandic-quality)
• á is a clear “ow/au”-like long vowel (not like English “a”) in á þriðju hæð