Learn Icelandic: Mýrin er neðan byggingarreitsins. - The swamp is below the building plot.

The preposition neðan (below) belongs to a specific group of spatial prepositions that govern the genitive case. Because of this, the noun phrase that directly follows it must take the genitive form.

How do you break down the compound word byggingarreitsins?

It is formed from bygging (building) and reitur (plot or square). In Icelandic, the first word of a compound is often locked in the genitive case (byggingar), while only the final word declines to fit the sentence. Here, reitur becomes reits to satisfy the genitive requirement of neðan, followed by the attached definite article -ins.

Could I use fyrir neðan instead of just neðan in this sentence?

Yes, and in everyday spoken Icelandic, fyrir neðan is actually much more common. However, be careful: while neðan on its own takes the genitive, the compound preposition fyrir neðan takes the accusative case. If you used it here, the sentence would be "Mýrin er fyrir neðan byggingarreitinn." Using neðan alone sounds a bit more formal, which fits the context of land plots or official documents.

Why is the definite form of mýri (swamp) written as mýrin instead of "mýriin"?