Learn Icelandic: Frumkvæði er einmitt eiginleikinn sem okkur vantar. - Initiative is exactly the quality that we need.

The verb vanta (to lack / to need) is an impersonal verb. It requires the "subject" (the person who needs something) to be in the accusative case, not the nominative. The accusative form of við (we) is okkur.

If vanta requires the thing being needed to be accusative, why is eiginleikinn nominative?

Eiginleikinn (the quality) gets its nominative case from the first half of the sentence, where it follows the verb er (is). In the second half, the relative particle sem (that) invisibly steps in to act as the accusative object for vantar.

This lesson covers demonstrative pronouns like "sá sem" and "það sem". Why is only sem used here?

Phrases like sá sem (the one who) or það sem (that which) act as placeholders when a noun is omitted. Because we specifically name the noun here (eiginleikinn), we just link it with sem. If we dropped the noun, we would use a demonstrative pronoun: Frumkvæði er einmitt það sem okkur vantar (Initiative is exactly that which we need).

How does the word eiginleikinn break down?