Learn Icelandic: Af hverju borðar þú þennan holla mat?

Af means "from" or "of," and hverju is the dative form of hvað (what). So literally, it means "from what" or "of what" (as in, "from what cause"). This is the standard way to ask "why" in Icelandic.

Why is the verb borðar before the subject þú?

Icelandic uses V2 (verb-second) word order. Because the question phrase Af hverju takes up the first slot in the sentence, the verb borðar must come next in the second slot, pushing the subject þú to the third position.

Why is the word for "this" þennan instead of þessi?

The food (matur) is the direct object of the verb borðar (to eat), so it must be in the accusative case. Matur is a masculine word, and its accusative form is mat. The word for "this" has to match it, so the masculine accusative form of þessi becomes þennan.

Why is the adjective for healthy holla instead of hollur or hollan?

Because it comes right after a pointing word like "this" (þennan), the adjective has to use its "weak" form. For masculine accusative words like mat, the weak adjective ending is an -a, making it holla. If you didn't have the word "this", you would use the strong form: hollan mat (healthy food).