Questions & Answers about Sérð þú pósthúsið sem er undir svölunum?
Why is svölunum in the dative case here?
The preposition undir (under) takes the dative case when describing a fixed location, like where a building is situated. If something were actively moving to get under the balcony, it would take the accusative case instead.
"Balcony" is singular in the English translation, so why does svölunum have a plural ending?
In Icelandic, the word for balcony (svalir) is a feminine noun that only exists in the plural form, even if you are only talking about a single balcony. Since it follows undir here, it takes the dative plural definite ending -num, becoming svölunum.
Is pósthúsið in the nominative or accusative case here?
It is in the accusative case because it is the direct object of the verb sjá (to see) — it is the thing being seen. However, because pósthús is a neuter word, its accusative form looks exactly the same as its nominative form. The -ið at the end is just the attached word for "the".
Does the word sem (that) change depending on what it refers to?
No, sem is a very convenient word! Whether it translates to "who", "which", or "that", it never changes form to match gender, number, or case.