Learn Icelandic: Strákurinn er spenntur og hlakkar til jólanna. - The boy is excited and looks forward to Christmas.

The verb phrase hlakka til (to look forward to) requires the following noun to be in the genitive case. The preposition til is what forces this change. jólanna is the genitive, definite form of the word jól.

Why does jólanna have a plural ending? Is he looking forward to multiple Christmases?

No, just one! In Icelandic, the word for Christmas, jól, is a neuter plural noun. It is always used in the plural, even when talking about a single Christmas season. The -nna part is the definite plural genitive suffix.

Would spenntur change if the subject was a girl?

Yes, it would. Adjectives like spenntur (excited) must agree with the gender of the noun they describe. Since strákurinn (the boy) is a masculine noun, the adjective is masculine. If the sentence was about stelpan (the girl), you would use the feminine form spennt.

Do we need a special case for the person who is looking forward to something?

Good news: no! Even though many emotion verbs in Icelandic use "quirky subjects" (where the person feeling the emotion is in the accusative or dative case, like with langa), hlakka til takes a standard nominative subject. That is why we use the dictionary form strákurinn.