Learn Icelandic: Hann er einn af snjöllustu vísindamönnunum.

The preposition af always requires the dative case. In partitive constructions like einn af (one of), the entire group that follows the preposition must be dative, giving us the plural dative form vísindamönnunum.

Why does vísindamönnunum have the definite article attached when the English translation just says 'scientists'?

In Icelandic, when you use einn af to say 'one of a group', the noun for that group usually takes the definite article. You are essentially saying 'one of the cleverest scientists'. The suffix -unum is the dative plural definite ending.

Why does the superlative adjective snjöllustu end in -u?

Because it is in the weak declension. Whenever an adjective is followed by a noun with the definite article attached, it must be weak. In the plural, all weak adjectives end in -u, regardless of their gender or case. The U in the ending also triggers a U-shift, changing the A in the root word snjall to an Ö.

If we were talking about a female scientist, would einn change?

Yes! The word for 'one' must agree with the gender of the subject. If the subject were female, you would say Hún er ein af... because ein is feminine. However, the noun vísindamaður (scientist) is always grammatically masculine, so the rest of the sentence (snjöllustu vísindamönnunum) would stay exactly the same.