Questions & Answers about Af hverju notar þú hunang í stað sykurs?
Why is the word for sugar sykurs instead of just sykur here?
The phrase í stað means "instead of" (literally "in place"), and it is a complex preposition that requires the noun following it to be in the genitive case. The genitive form of the masculine noun sykur is sykurs.
If í stað triggers the genitive case, why is hunang not in the genitive too?
Because hunang (honey) is not governed by the preposition í stað. It is the direct object of the verb nota (to use), which takes the accusative case. Hunang is a neuter noun, so its accusative form looks exactly the same as its dictionary form.
Could I use the phrase í staðinn fyrir in this sentence instead?
Yes, you could! In fact, í staðinn fyrir is more common in spoken, everyday Icelandic. However, because the preposition fyrir takes the accusative case here, you would drop the genitive -s on sugar: Af hverju notar þú hunang í staðinn fyrir sykur? The phrase í stað is slightly more formal but very useful to know, especially in writing.
I know Af hverju means "Why", but what is happening grammatically in that phrase?
It literally translates to "from what" or "of what". The preposition af requires the dative case. Hverju is simply the dative neuter form of the question word hver (who/what/which). Even though you learn it as a set chunk early on, it perfectly follows the rules of prepositional cases!