In Icelandic, when you are moving towards a destination—like going into, onto, or around a place—the prepositions you use will force the following noun into the accusative case.
Á (Onto / Up on)
Let's start with the preposition á (onto). We will also see the verb ganga (to walk). Since fjallið (the mountain) is a neuter noun, its definite form looks exactly the same in both the nominative and accusative cases.
Um (Around / Through)
The preposition um means "around," "through," or "about." It always takes the accusative case. Notice what happens to masculine nouns: the nominative bærinn (the town) becomes bæinn, and garðurinn (the park) becomes garðinn. Let's pair it with the verb keyra (to drive):
Í (Into / To)
When you are going into or to a specific place, you use í. For feminine definite nouns, the accusative ending changes from -in to -ina. For example, sundlaugin (the swimming pool) becomes sundlaugina.
More Practice with Motion
Let's look at a few more examples combining our verbs of motion with á, um, and í. Here, you will see vegurinn (the road) and skógurinn (the forest) in their accusative forms (veginn and skóginn).