Póstkassinn er við hurðina, en glugginn á svölunum er opinn.

The mailbox is by the door, but the window on the balcony is open.

Breakdown of Póstkassinn er við hurðina, en glugginn á svölunum er opinn.

• vera: to be
• opinn: open
• á: on
• en: but

Questions & Answers about Póstkassinn er við hurðina, en glugginn á svölunum er opinn.

Why do several nouns have endings like -inn, -ina, and -unum attached to them?

Those endings are the Icelandic definite article, which is usually added to the end of the noun instead of being a separate word like English the.

So here:

• póstkassi = mailbox

• póstkassinn = the mailbox

• hurð = door

• hurðina = the door

• svalir = balcony / balconies
• svölunum = on the balcony / balconies, in dative plural definite form

The exact ending changes depending on gender, number, and case.

Why is hurðina in that form after við?

Because við normally takes the accusative case in modern Icelandic.

So:

• hurð = door
• hurðina = the door, accusative singular definite

In this sentence, við hurðina means by the door or next to the door.

Why is svölunum dative after á?

The preposition á changes case depending on meaning:

• á + accusative often shows motion toward something
• á + dative often shows location in/on something

Here the meaning is static location: the window is located on the balcony, not moving onto it. So Icelandic uses dative:

• á svölunum = on the balcony