Learn Icelandic: Ég sé framherjann skora eitt af fallegustu mörkunum. - I see the striker score one of the most beautiful goals.

The word for "one" has to match the gender of the items you are talking about. Here, "one of the goals" refers to mark (a goal), which is a neuter noun. Therefore, you must use the neuter form eitt. If you were talking about "one of the cars" (bílunum, masculine), you would use einn af.

Why are fallegustu mörkunum in the dative case?

In Icelandic, the preposition af always dictates the dative case. Because this partitive construction uses af to say "of the most beautiful goals," the words that immediately follow it must be in the dative plural, regardless of what the rest of the sentence is doing.

Why is the verb skora in the infinitive instead of conjugated?

This sentence uses a structure that works exactly like English! When you see, hear, or let someone do something, you use the accusative case for the person or thing doing the action (framherjann) and the infinitive for the action itself (skora).

Why does the superlative adjective fallegustu end in -u?

Because it modifies a noun with the attached definite article, mörkunum (the goals). When an adjective comes before a definite noun, it must use the weak declension. In the weak declension, all plural superlative adjectives end in -u, no matter their gender or case.