Learn Icelandic: Lestin fer í gegnum löng göng. - The train goes through a long tunnel.

In Icelandic, the word göng (tunnel or corridor) is always a plural noun, even when referring to just one single tunnel. Because it is a neuter plural noun, the adjective modifying it must also be in the neuter plural form, which is why we use löng instead of the singular langt.

What case does the phrase í gegnum require?

The prepositional phrase í gegnum (through) always requires the following noun to be in the accusative case. Therefore, löng göng is in the accusative case here, though for neuter words like göng, the nominative (subject) and accusative (object) forms happen to look exactly the same.

Why does the adjective langur (long) become löng?

To match the neuter plural accusative noun göng, the root adjective langur takes a hidden ending that triggers a common vowel shift known as u-mutation. This rule changes the root vowel 'a' into 'ö', turning the word into löng.

How would you say 'through THE long tunnel' instead of 'a long tunnel'?

To make the phrase definite, you would add the definite suffix to the noun and use the weak form of the adjective. The phrase would change from í gegnum löng göng to í gegnum löngu göngin.